This article is not a part of the Warriors universe and is from the real world.
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 comes from the original source or author of the fictional universe. Spin-offs and adaptations to other media are often non-canon, and fanfiction is always non-canon, due to it not being published or released by anyone associated with the the official franchise.
All books in the Warriors series are canon, as they've all been released by HarperCollins under the Warriors franchise.Warriors graphic novels are considered canon as well, despite mainly being released by TokyoPop, because they are officially sanctioned and published.
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 she had nothing to do with their creation.