Canon, in the context of a fictional universe, comprises those novels, stories, films, etc., that are considered to be genuine or officially sanctioned, and those events, characters, settings, etc., that are considered to have existence within the fictional universe. In order for a setting to appear coherent, 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.

Material that is considered canon usually comes from the original source or author of the fictional universe, while (some) spin-offs and adaptations to other media are more likely to be non-canon and fanfiction is always non-canon, due to it not being published and released by anyone associated with the Warriors franchise.

Canonicity in Warriors

All books in the Warriors series are canon. The Field Guide books, such as Secrets of the Clans, are canon as well, since, even though they are spin-off, they detail facts about the Warriors series, which are known to be true.
There have been disputes as to whether or not the Official Site's family trees are canon or not, but it has been confirmed several times by Vicky Holmes that they are false, and that she had nothing to do with their creation.[1][2]

References and Citations

  1. Revealed on a Wands and Worlds chat here
  2. Revealed on a Warriors Wish interview here