Adapted from Wikipedia:Copyleft:
Copyleft is that property of a human creation (or discovery) that aims at preventing the use, the propagation, and the modification of that creation (or discovery) from being hindered by Intellectual Rights.
One of the most important reasons why creators or authors might want to make copyleft applicable to their work is that in doing so they hope to create the most favourable conditions for a wide range of people to feel invited to contribute improvements and/or elaborations to this work, in a continuing process.
In an interview published August 18, 2004 in Business Week, Linus Torvalds draws the attention to the fact that such approach without impediments for spreading knowledge is the normal way for progress in science (i.e. when using the scientific method): always building further on what others have discovered before, not being hampered by secrecy surrounding prior research results.
Methods for copylefting
Common practice for achieving this goal of hassle-free exploitation, copying and distribution of a creation or a work (and its derivates) is to ship it with a license. Such license would have to stipulate that every owner of (a copy of) the work can:
- 1. use it without limitation;
- 2. can (re-)distribute it in as many copies as desired;
- 3. can modify it in any way as seems fit;
These three freedoms, however, do not yet assure that a work that is derived from the creation will be distributed under the same un-limiting conditions: in order for the work to be copylefted, the license has to make sure that the owner:
- 4. will only distribute the derived work(s) under a same type of license.
Other (additional or understood) license conditions that would take away possible impediments to the hassle-free use, distribution and modification of a work include:
- making sure that the copyleft license conditions can not be revoked;
- making sure that the work, and its derived versions, are always made available in a form that facilitates modification. E.g. for software, this facilitating form is considered to be a synonym to source code, where also the compilation of such source code should be guaranteed to be without impediments of any kind.
- devising a more or less compulsory system for properly documenting the creation and its modified forms, by way of user's manuals, descriptions, etc...
Most commonly such `copyleft` license, in order to have any kind of effect, would need to make creative use of rules and laws governing intellectual property. E.g. when hinging to copyright law (which is the case most often), all the persons that in any way contributed to the copylefted work will become (co-)copyright holders of the work, but at the same time, by subscribing to the license, will deliberately give up some of the rights that normally follow from copyright, for instance the right to be the unique distributor(s) of copies of the work.
While depending on laws governing intellectual property, which can be different from one country to another, the actual license, which is not more than a method for achieving the copyleft goals, can also be different from one country to another. For example in some countries it might be OK to sell a software product without warranty, in standard GNU GPL style (see art. 11 and 12 of the GNU GPL license v.2), while for instance in most European countries it is not possible for a distributor of software to waver all warranties regarding a sold product, for which reason the extent of such warranties are described in most European copyleft licenses (see e.g. the CeCILL license, a license that allows to use GNU GPL - art. 5.3.4 of CeCILL - in combination with a limited warranty - art. 9).
History - definitional issues
The concept of copyleft arose when Richard Stallman was working on a Lisp interpreter. Symbolics asked to use the Lisp interpreter, and Stallman agreed to supply them with a public domain version of his work. Symbolics extended and improved the Lisp interpreter, but when Stallman wanted access to the improvements that Symbolics had made to his interpreter, Symbolics refused. Stallman then, in 1984, proceeded to create a software license that would prevent this behavior which he named software hoarding. The term `copyleft` according to some source came from a message contained in Tiny BASIC, a free distributed version of Basic written by Dr. Wang in the late 1970s. The program listing contained the phrases `All Wrongs reserved` and `CopyLeft.` Richard Stallman himself says the word comes from Don Hopkins, who he calls a very imaginative fellow, who mailed him a letter in 1984 or 1985 on which was written: `Copyleft—all rights reversed.` 
There are definitional problems with the term `copyleft` which contribute to controversy over it. The term originated as an amusing backformation from the term `copyright`, and was originally a noun, meaning the copyright license terms of the GNU General Public License (GPL) originated by Richard Stallman as part of the Free Software Foundation's work. Thus, `your program is covered by the copyleft` is almost considered as a synonym of the program being GPL-ed (GPL being the de facto standard of all kind of copyleft licenses). When used as a verb, as in `he copylefted his most recent version`, it is less precise and can refer to any of several similar licenses, or indeed a notional imaginary license for discussion purposes. See also the next section, that goes in detail about some definitional issues.
Because of complications caused by use of software library routines, the FSF created the GNU Library General Public License (LGPL), which changes the requirement of further distribution in ways that make it possible to use a software library without being `infected` by its license. The LGPL was subsequently renamed to the GNU Lesser General Public License, in order to emphasize its implications and encourage software library authors to choose the ordinary GPL instead (also because LGPL is fundamentally not a copyleft license). 
Copyleft, Public Domain and Open Source - `strong`, `weak`, `full` and `partial` copyleft
No restrictions apply to works in the public domain. They may be freely modified, and the creator of the derivative work may license any new portions of the derivative work, but not the public domain portion, under any terms, or none. The resulting derivative work may not be available to the creators of the original or may compete with them. So, a public domain work is by definition not subject to full copyleft.
Copylefted and non-copylefted Open Source software
Copyleft is one of the key features distinguishing several types of open source software licenses (eventually copyleft became the key issue in the strife between the Open Source movement and the Free software movement): copyleft is the short name for a legal framework to ensure that derivatives of a licensed work stay free/open (which is not compulsory in a general `Open Source` approach). If the licensee of a copylefted work distributes derivative works not under the same (or in some cases: similar) copyleft license he will be facing legal consequences: for most copylefted works this implies at least that some provisions of the license are terminated, leaving the (former) licensee without permission to copy and/or distribute and/or publicly display and/or prepare derivative works of the software, etc.
Many open source software licenses, such as those used by the BSD operating systems, the X Window System and the Apache web server, are not copyleft licenses because they do not require the licensee to distribute derivative works under the same license. There is an ongoing debate as to which class of license provides a larger degree of freedom. This debate hinges on complex issues such as the definition of freedom and whose freedoms are more important. It is sometimes argued that the copyleft licenses attempt to maximize the freedom of all potential recipients in the future (freedom from the creation of proprietary software), while non-copyleft free software licenses maximize the freedom of the initial recipient (freedom to create proprietary software). It can also be seen as distinguishing the freedom of the individual software authors from the freedom of the software itself.
Strong and weak copyleft
The copyleft governing a work is considered to be `stronger`, to the degree that the copyleft provisions can be more efficiently enforced to all kinds of derived works. `Weak copyleft` is sometimes no more than an euphemism indicating that the work has no real enforcable copyleft.
An example of a free software license that uses strong copyleft is the GNU General Public License. Free software licenses that use `weak` copyleft include the GNU Lesser General Public License and the Mozilla Public License. Examples of non-copyleft free software licenses include the Q Public License, the X11 license, and the BSD licenses.
Full and partial copyleft
`Full` and `Partial` copyleft relate to another issue: Full copyleft is when all parts of a work (except the license itself!) can be modified by consecutive authors. Partial copyleft implies that some parts of the creation itself are made exempt of unlimited modification, or in another way not completely subject to all principles of copylefting. E.g. in artistic creation full copylefting is sometimes not possible or desirable (see below).
Copyleft licenses are sometimes referred to as viral copyright licenses, often by those who feel that they may lose out as a result, because any works derived from a copylefted work must themselves be copylefted. The term `viral` implies propagation like that of a biological virus through an entire organ of similar cells or species of similar bodies. In context of legally binding contracts and licenses, `viral` refers to anything, especially anything memetic, that propagates itself by attaching itself to something else, regardless of whether the viral assertions themselves add value to the individual work. The viral metaphor is over-used but is reasonable to help distinguish between free software and open source in software and documentation projects. Most advocates of copyleft argue that the analogy between copyleft and computer viruses does not apply. As they point out, computer viruses generally infect computers without the awareness of the user, whereas the copyleft actually grants the user certain permissions to distribute modified programs, which is not allowed under copyright law without permission of the copyright holder. Most proprietary software licenses do not allow such distribution. Furthermore, copyright itself is `viral` in this sense, since any works derived from a copyrighted work must have permission from and obey any conditions set by the original copyright holder.
The view that copyleft licenses are viral is supported by Microsoft, who say that if a product uses GPLed code, that product automatically escapes the creator's control and becomes GPLed, leaving the creator no recourse. Obviously, working for a software company will have a like effect. Advocates, including Eben Moglen, Professor of Law at Columbia University and counsel for the Free Software Foundation, note that this is not true since the GPL is a license, not a contract.
Microsoft, and others, in describing the GPL as a `viral license`, may also be referring to the idea that any release of something new under the GPL would seem to create a positive feedback network effect, in which over time there will be an ever-expanding amount of copylefted code. Code reuse is often useful in software engineering, as a way to save effort and get on with a project, especially when a perfectly sensible design and implementation has already been done and is available. In contrast, those working on non-copylefted programs will have to `reinvent the wheel` for parts of their programs. This is often cited as a disadvantage of non-copylefted software development.
Many feel that copyleft licenses are desirable and popular for shared works precisely because they are viral, and apply to all derivative works, which are thus `infected` by the requirement to re-integrate changes deemed desirable by any party down the line. This requirement is seen as important because it ensures uniform license terms and free access, and makes copyleft projects resistant to unnecessary forking because all maintainers, of the original work or other versions, may use any modifications released by anyone. Useful changes tend to be merged, and different versions are maintained only to the extent that they are useful. Without the `viral` license, variant terms can apply to the forks and derivative works can be controlled commercially by the parties that extend or translate them, which can be considered as some of the disadvantages of non-copyleft `open source` projects. It is thought that Linux has not suffered the same fragmentation as Unix because it is copylefted.
Copyleft applied outside the context of copyright licensing for software
Art - documents
Copyleft also inspired the arts (especially where traditional notions of intellectual property are experienced as hampering creativity and/or creative collaboration and/or easy distribution of art creations), with movements like the Libre Society and open-source record labels emerging. E.g. also the Free Art license is a copyleft license that can be applied to any work of art.
Copyleft licenses for materials other than software include the Creative Commons ShareAlike licenses and the GNU Free Documentation License (abbreviated to GNU FDL or GFDL or FDL). GFDL can be used to apply copyleft to works that have no distinghuishable source code (while the GPL definition relies on source code being distinghuisable from compiled code or object code or executable code or binary code in a work). GFDL is being used for the content of Wikipedia.
Note that the notion of copyleft, to make sense, somehow requires a space where hassle-free & cheap copying is common (...computer files or Xerox copies, etc), or, to put it otherwise, where one can give away without `losing` what one is giving away (like knowledge): e.g. copyleft is more difficult to put in practice for those arts that are characterised by the production of unique objects, that can not be copied just like that - unless there is no fear of the unique original getting damaged. To illustrate this with an example: suppose there is a public display of some worldfamous paintings, e.g. some of the many copies and derived works Andy Warhol (had) made of his own art works, and suppose an accidental visitor decides to `enhance` these paintings with some grattage and peinture brulÃ©e effects (not neglecting to sign his respectful contributions with some spray paint), then there is no (legal) way of stopping this guy if he can be considered copyleft licensee of these paintings. This - and other examples - may indicate that copyleft is not the ultimate philosopher's stone that would be able to solve all intellectual property related issues once and for all: especially in art, that also has a tradition of creation as a solitary process (along with, but quite separated from, a tradition of cooperative creativity), `community drive` is not in all cases desired.
Copyleft licenses for art generally are aware of such limitations, so they differ from copyleft licenses for software, e.g. by making a distinction between the initial work and the copies (where some essential copyleft provisions are only applicable to the copies) and/or by leaning on notions that are less objective to put in practice (more like declarations of intent), for example stipulating copyleft to be subject to respect - in a programmers' world the implementation of copyleft itself is the maximum respect one can get. In other words: in art copyleft has to hinge on broader notions regarding authors' rights, which are even more complex (and more differing between countries) than mere copyright law, see e.g. Moral rights, droit d'auteur, Intellectual Rights and Berne Convention for the Protection of Literary and Artistic Works.
Whether used for creation of art or in a technical context, GFDL allows similarily that authors use limiting techniques regarding their initial work, exempting some parts of their creation from the full copyleft mechanism, e.g. introducing `invariant sections`, etc... (these techniques are however not used for the Wikipedia GFDL).
Copyleft-like ideas are also increasingly being suggested for patents (so, hinging on a patent law framework instead of on a copyright law framework), such as open patent pools that allow royalty-free use of patents contributed to the pool under certain conditions (such as surrendering the right to apply for new patents that are not contributed to the pool).
Since for most copylefted creations the copyleft characteristic is however only secured by copyright law, patenting mechanisms can threaten the copyleft freedoms attached to such creations, when patent law is allowed to overrule copyright (or in any other way limit the free expansion of copylefted creations), which might be the case for the new rules regarding patents developed in the European Union in the early 21st century (see also article on Community Patent). There seems to be no easy answer to such threats, while it is considered that generally communities developing copylefted products have neither the resources nor the organisation for complex patenting procedures. However, an organised answer to such issues seems to start emerging from places like Groklaw. Also IBM could be considered by the open source community as rather an ally, when it comes to combining traditional copyright protection for copylefted creations with patented inventions, see: Infoworld article notifying that IBM says it won't assert patents against Linux kernel
Commercial exploitation of copylefted creations
Whether art or industrial realisation, the commercial exploitation of rigourously copylefted creations has rules that are quite different from a traditional commercial exploitation via Intellectual Property Rights (IPR). Several research projects (amongst which some by the EU, e.g. the FLOSS project), show that in order to make business with Open Source Software either the copyleft aspect gets somehow overruled (e.g. by making money of temporary know-how advantages), either the business is developed exclusively on a model of services and/or consultancy surrounding the actual copylefted creations. Generally turnover (when expressed in financial terms) is expected to be much lower in a `copyleft` business than in a business that tries to exploit the sale of IPR. E.g. John Cage, and his heirs, would never have been able to make much money with the sale of printed music and public performance rights of the 4′ 33″ composition (which, from a commercial viewpoint, is, to put it exactly, hot air served at room temperature), plus throwing in an occasional lucrative litigation, weren't it for traditional exploitation of IPR.
Development of copylefted industrial products
In this sense, the competitive position of businesses based on copylefted industrial products may at first sight seem excessively weak, while not being able to create large financial resources for e.g. research and development, neither having exclusivity for the profits gained with the result of such research and development: nonetheless it has been argued that copyleft is about the only mechanism able to compete with really big players that are for a large part depending on financial exploitation of IPR: see e.g. The Cathedral and the Bazaar, where Eric Raymond, backed by Linus Torvalds, argues (amongst other things, and on this point maybe only indirectly) that a competitional strength of copylefting can be that programmers feel even more involved in their creation (while there is a relatively easy system warranting that any future derived product will witness their contribution, and will remain accessible to them), and that that extra involvement can contribute to extensive (and frankly, rather abstract) projects like the development of the kernel of an OS. In the words of Linus Torvalds, trust appears to be the central issue here: while rigorously copylefting the Linux kernel, he makes clear his intent of never abusing (or hiding) any knowledge that is contributed to it, and thus he became the most trusted party for many small and large contributors. See also argumentation with regard to copyleft being better in avoiding uncontrollable forking, given above, which also appears to be an advantage in the Linux development process. Ultimately, the GPL-ed Linux would become, more than any other open source OS, like BSD UNIX, real competition to the MS DOS/Windows line of operating systems - which could be considered as quite a revolution.
Commercialisation of copylefted industrial products
Commercial distributors of Linux-based systems (like Red Hat and Mandrake) might have had some ups and downs in finding a successful construction (or Business Model) for setting up such businesses, but over time it appeared to be perfectly possible to base a business on a `commercial service surrounding a copylefted creation`, see e.g. Mandrake's success on the stock market (one of the first after the implosion of large parts of the IT market in the early 21st century) and their success in convincing governement bodies to switch to their flavour of Linux. Note however that, apart from rare exceptions like Debian, most of the Linux distributors don't limit their business to copylefted software. Although there appears to be no real reason why an exploitation of `commercial services surrounding copylefted creations` would not be possible in small-scale business (which as a business concept is probably even simpler than making money with a `public domain` recipe for brewing coffee - successfully exploited by so many cafeteria owners), there are maybe not yet much examples of SMEs having risked such jump for their core business.
Commercialisation of copylefted art
In art the concept of a `commercial service surrounding a copylefted creation` is maybe (even) harder to put in practice than in software development. Public performances could be considered as one of a few possibilities of providing such `services`. Apart from the fact that not every artist is a born performer, an artist generally expects at least some kind of respect for his/her work (which is somehow undermined if applying the copyleft condition undiluted, i.e. without applying the additional conditions regarded as self-evident in a `conventional` artistic context, e.g. not mingling your own whistling to the sounds produced by a chamber ensemble in a concert - while rigourously copylefting a work without imposing additional conditions implicates renouncing to the right to see any deformation of that work as disrespectful, as long as it is known who operates the alterations). The conditions an artist has to live up to under strict copyleft licensing have something ruthlessly and self-denyingly adventurous, comparable to what Wikipedia has in bold on every `edit` page: `If you do not want your writing to be edited mercilessly and redistributed at will, do not submit it.`
Often copylefted artistic creations can be seen to have a (supporting) publicity function, promoting other, more traditionally copyrighted creations by the same artist(s). Artists sticking to an uncompromising copylefting of the whole of their artistic output, could, in addition to services and consultancy, revert to some sort of patronage (generally considered as somehow limiting artistic freedom, even when state-operated like in some countries of northern Europe), or to other sources of income, not related to their artistic production (and so mostly limiting the time they can devote to artistic creation too). The least that can be said is that copylefting in art tends toward keeping the art thus produced as much as possible out of the commercial arena - which is considered as an intrinsic positive goal by some.
- Copyleft and software:
- www.gnu.org: What is copyleft? (note: although afterwards the GNU copyleft licences were applied in other fields, this article is basicly written from the viewpoint of computer programs)
- a Norwegian Copyleft Software website
- a European report (2000) - Report includes section about possible economic models for Free/Libre and Open Source Software.
- Why Free Software's Long Run TCO must be lower - An economic analysis of copylefted software and the software market.
- Copyleft applied in artistic creation: