In the context of fiction, the canon of a fictional universe comprises those novels, stories, films, etc. that are considered to be genuine (or "official"), and those events, characters, settings, etc. that are considered to have inarguable existence within the fictional universe. In order for the fictional universe to appear cohesive, especially in fictions that contain multiple parts, both creators and audiences sometimes find it useful to define what has and has not "actually happened" in that universe. Usually items that are considered canon come from the original source of the fictional universe while non-canon material comes from adaptations, spin-offs or unofficial items, often in different media.
While every single player-made character could be considered non-canon, the characters are what we would like to consider "Community-Canon," making them canonical in the Warcraft community. Non-canon would then, in turn, be considered non-community-canon.
Things that would be considered non-canonEdit
Relation to Campaigns Other Than WarcraftEdit
An example of relation to campaigns other than Warcraft would be tying in campaigns such as Lord of the Rings, Forgotten Realms, Greyhawk, Warhammer, etc. into World of Warcraft characters, guilds, storylines, and so on.
This also is used for tying in real life into World of Warcraft. If your character is supposed to be from Earth, Jupiter, or is related to Bill Gates (to name a few examples,) then it is non-canon.
Relation to Lore FiguresEdit
This would be something such as saying that you are the brother of Maiev, sister of Furion, daughter of Khadgar, etc. This may also count for saying that you are the brother to Marshal McBride or mother of Auctioneer O'Reely.
One exception would be if you are a night elf and you say that you are a daughter/son of Cenarius. Also, non-biological relations are accepted (for example, a warden that worked with Maiev, or an elf that worked with Alleria,) however, you can't be Thrall's nanny or Jaina's babysitter.
Vampires, Dragons, Death KnightsEdit
There are no vampires in Warcraft. There are dread lords, and no player is a dread lord. Dragons are in limited number in Warcraft. If one person sees another dragon, everyone will want to be a dragon. Death Knights are also very limited, and all the Death Knight-player-characters we've seen just casually cruise through Goldshire or Stormwind like it's no big deal.
Basically, try not to stretch what your character is too far. If you are an Undead Female Priest that will get Shadow Form and say that you are a Banshee, then that's fine. Another acceptable case is saying that you're a tiger and you're a night elf/tauren druid that's always in cat form and doesn't talk. There are some debatable things such as trolls being night elves, orcs being half-orcs, and night elves being undead elves (though such cases should generally be accepted and considered canon.) However, if you are an undead and say you're a crypt fiend, or a human and say you're a dread lord, or a troll and say you're a bat or a vampire, dragon, or death knight, then it's non-canon.
You can't be the founder of Stormwind, founder of magic, first person to ever visit Winterspring, first Blood Elf to die from Maiev, only escaper from the Dark Portal just as Khadgar and the others closed it, only warden survivor other than Maiev in Illidan's cell as Tyrande killed everyone, or only survivor other than Maiev in Sargeras's tomb as Illidan destroyed the exit.
You also can't say, "Maiev was going to kill me, but intentionally let me go." While these things can limit one's idea of a character concept, it actually encourages you to use your imagination when making one.