Fanon

Fanon has several definitions, and is a term that applies to almost every media-based or Internet fandom. As defined in the TV Tropes Wiki, fanon is "the set of assumptions based on that material which, while they generally seem to be the 'obvious' or 'only' interpretation of canonical fact, are not actually part of the canon. Occasionally, the explanation seems good enough to just be 'common sense.'" In other words, fanon is what the fans think fills in the gaps in the original show. Fanon becomes especially potent when put into writing and referenced thereafter.

Some people think that canon is what is "true" or "real" about a TV series, and fanon is everything that is not. This is not correct. Fanon exists to support the reality created by canon, not to contradict it.

Daria Fanon
In Daria fandom, fanon might mean:

1. Anything said or implied about Daria that is not strictly canon, but does not contradict canon and is therefore widely accepted by most fans. The source of such material often has a semi-official tone to it. Many (but not all) fans have concluded that Highland was located in Texas, based on comments in an interview with the creator of Beavis and Butt-head, Mike Judge, and that the Morgendorffers moved out of Texas to Lawndale, located in another state like Maryland or Pennsylvania but not Texas, based on an interview with Daria creator Glenn Eichler.

2. Any fan-created character, device, location, or event popular enough to be used by other fans in their own works. In Daria fanfiction, examples of shared characters used by multiple authors include Brother Grimace's Kyle Armalin, Canadibrit's Lynn Cullen, Scissors MacGillicutty's Jim Vitale, Robert Nowall's Veronica Morgendorffer, and TAG's Scarlett. (Note that these characters are most commonly seen in alternate-universe fanfiction.) Some unofficial characters could, at a stretch, be considered part of a Daria Expanded Universe, as long-time fanfiction readers know the characters so well that they can joke about their foibles and personality characteristics without feeling the need to explain to newcomers who those characters are.

In addition, fans have given Daria the middle name Anne, Louise, or Marie, each of which has been copied by other fans in their writings. None of these conventions explicitly defies canon, and could therefore be considered "fanon."

The term "fanon" originally consisted only of the first definition, but in this fandom has expanded to include the second definition above.

Jossing
An important aspect of fanon is when those commonly held assumptions about a show are contradicted by later official material, especially if aired on TV. This is called Jossing, after Joss Whedon, the creator of Buffy the Vampire Slayer. (See the link for a full explanation.) Early Daria fanfics were sometimes Jossed, but now that the series has ended it seems nothing can be so discounted, so our assumptions about Daria are (sadly) safe. In fact, wide use of the alternate-universe convention within this fandom has weakened potential Jossing to the point that most stories that diverge from canon can be enjoyed on their own merits because "it all takes place in another Dariaverse." The longer the original series remains off the air, the more authority is given by fans to other fan creations, and the stronger fanon becomes in the minds of all. While this might place the acceptance of canon at some risk, the fandom does what it needs to do in order to survive and prosper.