Canon is a term derived from Biblical scriptures, and in fiction terms refers to the material that 'counts'.
The term was originally devised by Sherlock Holmes fans, to distinguish the original Conan-Doyle stories from those by other writers: the original stories are the scripture and anything else written is derivative.
A second meaning for canon came along in later years: when the creator or rights holder declares that part of the official fiction counts and another part does not: for example, CBS Entertainment count only the Star Trek TV shows as canon. In the second instance, canon can be changed: Trek creator Gene Roddenberry declared The Animated Series wasn't canon, but in 2007 CBS declared that it was.
MTV on Daria's canon
MTV has never made an official statement on canon for either Daria or Beavis and Butt-head, and neither has Daria producer Glenn Eichler. In an interview with Kara Wild, when talking about avoiding giving real-time dates for a TV series, said "the only purpose it serves is to get people upset about 'violating canon'", giving the impression that an official canon was not a priority for him.
Anne D. Bernstein, however, jokingly referred to a canon in an interview, when talking about The Daria Diaries book: "I had to make up a ton of stuff that did not yet exist... and then it all became part of "the Daria Universe" so people had to follow what I established!".
In the absence of an official ruling beyond Bernstein's comment, "canon" is something that fandoms will make up informally. Since it's impossible to get all fans to agree, it's mostly down to personal preference (and, for fanfiction writers, whichever works best for the fanfic they're writing) and bursts of common sense. Alternatively, everything can be counted as equally canonical.
Official material and canon
Other official Daria material, all licensed by MTV, includes:
- The 65 episodes and two movies of Daria
- The pilot
- The two books on the series: The Daria Diaries and The Daria Database
- The "Daria's Inferno" computer game and "Daria's Sick, Sad Life Planner" CD utility
- Any materials presented in the Daria section of the MTV website
- Daria and Jane’s appearances as TV show hosts (off-canon canon) or in other media (magazine and TV "interviews," newspaper articles, etc.), excluding appearances that were not officially sponsored by MTV (see “In the Media” on Outpost Daria, via Wayback Machine)
- The Daria GPS Voice
- Any appearance of Daria in Beavis and Butt-head
- Any appearance of Daria in licensed materials for Beavis and Butt-head (books, computer games, material on the MTV website, etc.)
- The script to the Mystic Spiral pilot episode as released on Daria: The Complete Animated Series
The show itself is the bit that's taken to be definately, unarguably canon, as it's the core material and the basis of the fandom. Except when it isn't. A large number of fans do not see the episodes "Depth Takes a Holiday" and "Daria!" as canonical, as they are fantastic stories that clash with the usual format and tone of Daria; fantastic moments in episodes have also been scrubbed out by some fans. Despite this division, fans do all agree that the bulk of the show is fully canon.
The two books strongly support the background offered in the TV series, giving highly important supplemental information like the existence of Jodie's sister and Angela Li's political opinions. Crucially, they are written by Bernstein and future story editor Peggy Nicoll, and they both create material that would later appear on the show itself: the names of Helen's sisters, the details about Kevin's parents, and so on. Bernstein has also stated that other writers had to follow what she wrote. As a result of all this, the books are generally considered canonical. (A few items offered in the books contradict the episodes, like calling the Tommy Sherman Memorial Goalpost the "Tommy Sheridan" goalpost - a list of discrepancies is given in the section on Off-Canon Canon.)
The material on the MTV websites are again written by Bernstein and others, and tie in quite strongly to both the show and the books. They also presented information that was later called canonical by the MTV staff, such as the last names of Tiffany and Stacy (Blum-Deckler and Rowe, respectively). Again this is normally taken as canon by fans, and again there are some discrepencies with the show.
"Daria's Inferno" is almost entirely about a dream Daria has, so the only part that can be canonical is "Daria had a dream".
Beavis and Butt-head and its associated material is where the wheels come off a bit. This was where Daria Morgendorffer originated, and that means it's often taken as canon. However, the character was merely an occasional supporting character and was written as a foil for Beavis and Butt-head, and this means in some episodes and comics her personality is different to what we'd expect. In others, it's quite accurate. In the former's case, Daria is utterly blase about guns being fired around her in "Incognito". Daria also downplayed its connection to Beavis and Butt-head, and any time a younger Daria was seen in flashback material she was depicted as dressing similarly to her Daria look rather than her B&B look. Whether Beavis and Butt-head is canon for Daria, and which parts of it should be canon, is more disputed than other official material.
The cast made numerous appearances in interviews, MTV specials, and bumpers during episode marathons where they broke the fourth wall and referred to themselves as cartoon characters. In addition to this, there were letter pages for the Beavis and Butt-head comics, feedback on the website, and a IRC chat following Is It Fall Yet? where Daria spoke to specific, real-world fans. Obviously, this cannot be canon.
However, these are almost always presented in character and in some cases they were used to get across canonical information: for example, the ages of Daria, Trent, and Quinn were mentioned during Daria Day. As a result, we call this "off-canon canon" and collect it under the "Daria as Cartoon Actress" bracket.
Interviews and Canon
Unless outright stated, something mentioned in an interview with MTV staff is not going to be canon. However, these comments do tell us about how the creators may have seen a character turning out or tidbits that they'd never fully got into the show. A number of these have been taken as canon by many fans.
A key example is that in his interview with Kara Wild, Glenn Eichler gave the location of Lawndale as being in Maryland. The setting of Lawndale was never stated in canon and some parts of the show clash with a Maryland setting (a desert being so close for one), but since it's the nearest we'll get to an answer, Lawndale is taken to be a Maryland suburb in fanfic after fanfic. Daria's old hometown, Highland, was canonically placed in Texas in a 2011 Beavis and Butt-head, but even before then it was commonly assumed to be in Texas because Mike Judge had said that this was where he envisaged it being.
Canon and timelines
As stated at the top of the page, Glenn Eichler said he avoids giving hard dates to make the show easier to manouvre and to avoid the canon issue. As a result, there's no official line on timeline canon.
A few dates from the recent past had crept in to official material - Diary has Jake and Helen marrying in 1975, "Road Worrier" has Jake attending the 1969 Altamont concert - and 1990s/2001 dates have been given in episodes and spin-off media, but this was because the show was produced then. Whether, for the purposes of fanfic, you set the story in the past or have Daria in school in the present day depends on if you view it as canon that the show is set in 199x or as canon that the show is set "now" (Eichler's intention).