David Foster Wallace makes extensive use of Footnotes and Endnotes in his works.
Out of the two, footnotes are the most common way that Wallace inserts more information on a topic without having to put it in the main text. These appear, as indicated by the name, at the foot, or end of a page and are numbered, generally starting with 1, from the beginning of chapters or sections.
There are two extreme modes that Wallace uses for the footnotes, either there are a lot of them or they are extremely long.
One example of the first mode is his use of 128 separate footnotes in the last section of Everything and More.
In the second mode, the footnotes generally begin to take over the pages they start on and even spill over into the following page(s). These long footnotes tend to have information that is actually very important to understanding the story as a whole.
One of the worst (best?) examples of this is in the story Octet, in Brief Interviews with Hideous Men. Wallace uses some of the longer footnotes in this story in order to break the fourth wall so to speak and address the reader directly. In the Octet footnotes, he also uses the space to discuss how he failed to write the story he originally intended. Along with this, he includes stories in the footnotes that he started the footnote by saying he threw out.
In some cases, the footnotes themselves begin to have footnotes.
This abbreviation is short for "If You're Interested" and is used exclusively in Everything and More. Wallace uses this in order to try to indicate to the reader which footnotes are necessary to read in order to fully understand what he is saying and which are not. If he has to indicate that some footnotes or necessary, it brings up the question: "Are all footnotes necessary for understanding his other works?"
Out of all of Wallace's works, the only one that utilizes endnotes is Infinite Jest. The endnotes in IJ start at 1 (with a reference to meth) at the beginning of the novel and end at 388 (with a reference to another drug, Talwin) on the third to last page. Most of these endnotes are merely footnotes that have been relocated and collected in the back of the novel. Others however, have no business being called "endnotes," as they go on for pages and pages at a time--and in an excrutiatingly tiny font at that. Wallace uses these prolonged endnotes to add a second narrative. These endnotes, rather than make the reader's job easier (as shorter endnotes do by quickly explaining a reference), they make the reader's job that much harder, as they further dilate and obfuscate the already convoluted narrative.
The endnotes allow for a simultaneous sub-narrative, somewhat external to the text proper, and this allows Wallace to both assert narrative authority, as he controls the endnotes and places them where he sees fit; but also to subvert that narrative authority, especially with the longer endnotes, which draw the reader away from the text for extended periods of time, and editorializing endnotes, which demand the reader seriously question who the narrator or authoritative voice is.
In Infinite Jest, the process of flipping between the text and endnotes provides a physical example of Lyle's advice never to pull more than you weigh. The act of heaving the pages back and forth, back and forth, is very much like the bricklayer being pulled by the pulley's weight. As a reader, one could opt of of the endnotes, but if one is commited to the text, one must flip, and is thus pulled along by this weight. This is especially true in the first half of the novel, when the amount of pages one must flip exceeds the number one has read. The more one reads, however, the more one understands about the novel, and the less weight one has to grapple with to access the endnotes. The act of reading Infinite Jest is marked by profound introspection and effort on the reader's part, and it is a much-needed physical relief when the burden eases.