Canon

Fluff that is officially approved by whomever is in charge of that intellectual property. In essence, the notion that some made-up stories are more "real" than others.

It should be noted that canon ≠ cannon, no matter how frequently it is used as such. A lot of people confuse Canon with Continuity, Consistency or Consensus. Many state they have a "personal canon", or "headcanon". Sadly, that's not how canon works. Canon is what someone who is in charge of a particular fictional world tells you counts or doesn't count. Conflict arises when the canonical view doesn't match the consensus view. For example, Jar-Jar-Binks is part of the Star Wars Canon, because George Lucas says so. But the general consensus among Star Wars fans is to pretend that Jar-Jar-Binks never existed. This is commonly referred to as "fanon", as the consensus view is most often adopted by the average fans of a franchise, even if it directly contradicts the canon proper.

Canon itself can be sometimes unclear or subject to change, especially in works which have more than one author and have been poorly managed. The latter can be often seen in changes that occur between editions of a roleplaying game; for example in 3rd edition D&D the transformation of a Drow into the monstrous Drider was a punishment, in 4th edition it is a blessing the goddess bestows on the best of the society. When canon is changed in a way that counteracts previously established canon, it is called a Retcon. Retcons induce rage.

Doctor Who is an example of a work involving multiple authors where the shows producers have officially denounced the notion of canon, by stating; "It is impossible for a show about a dimension-hopping time traveller to have a canon." The show (and spin-offs) has continuity and consistency (such as how he's always played by a British man, though not the same actor, even though the Doctor can shape-shift into a person of any nationality or gender), but no official canon.

Games Workshop and canon

Games Workshop's official stance is that all of the fluff is told by an unreliable narrator, and is therefore true and false at the same time. This is so they can sell you multiple stories and products without having to wade through 25 years worth of bullshit, self-contradictions, and inconsistencies. This frustrates and annoys fa/tg/uys, who pull a fit every time their comic book collection gets even slightly out of order.

However, it's pretty common practice in most large franchises. This stance actually allows individuals to have their own 'personal canon', but of course, people don't see it that way and instead want their galaxy sweeping, massively scaled space opera to be detailed right down to how many pubic hairs Roboute Guilliman has.

Warhammer and Warhammer 40,000 have continuity and consistency to an extent, but nothing can be truly described as canon, as the powers that be never set anything in stone. Or to put it another way, Space Marines look a certain way and behave along certain guidelines, but exact details will vary greatly depending on who is telling the story.

On the other hand, some authors have truly taken the piss with this policy, such as Captain C.S. MULTI-LAZOR. However, even Games "everything is canon but not everything is true" Workshop have standards, and draw the line at his work; with one of their spokespeople saying it could've benefitted from an editor's red pen.

The /tg/ consensus

When it comes to things like Warhammer 40,000, fa/tg/uys tend to throw canon out the window and deal with the setting on their own terms, establishing their own continuity through group consensus, (the aforementioned fanon). Picking and choosing which retcons to accept and which to ignore is one of /tg/'s greatest pastimes, and to this end /tg/ has an established a board-wide consensus for almost any setting with more than one edition or author (all of them). This consensus tends to cross-pollinate with the stuff non-/tg/ fa/tg/uys also tend to like.

To again use Warhammer 40k as an example, an entire "secret" consensus has arose within the fandom, a continuity that "fixes" poorly received retcons, fixes consistency issues and tends to lighten the characters and infuse awesome into the setting wherever possible. /tg/ has even developed an entirely separate universe wherein fa/tg/uys can put all of their homebrew characters and factions.

Note that this a consensus, not canon. Canon is, in theory, what they say it is, whereas consensus is what the majority choose to accept. Consensus is also different from continuity, which in this case is about the look and feel of a series as a whole, rather than exact details.