In fiction, canon is the term used to classify truthfulness in a statement, or in an entire book, movie, short story, or television series. The opposite of canon is fanon, a mixture of the words "fan fiction" and "canon". Here is an example of a canon statement as opposed to a fanon statement:
Bob Gale's position on canon
When contributing to Futurepedia, please do not add fanon, but only material with canon sources. However, you may ask "What is a canon source?"
When asked by fans about the canon of anything outside the films, in response to a Telltale Games Question and Answer Session, Bob Gale specifically answered that "only the movies are canon. Everything else fits into 'what-if?' category, or might be canon in some alternate universe." Bob Gale Answers Your Back to the Future Questions
However, when addressing whether IDW's comics, which he co-wrote, were canon, he made a very different statement. In the first issue of IDW's Back to the Future: Citizen Brown, the comic adaptation of Telltale's Back to the Future the Game, Bob Gale stated "when you have a time machine and a premise that allows for multiple pasts, presents, and futures, everything is canon".
Contributing noncanon information
Only the three movies are absolute, unshakable canon. Deleted scenes and the novelizations are usually considered secondary canon by fans, if they do not conflict with what is seen in the films, but the source should still be indicated. The canonicity of the animated series, the theme park ride, and the Telltale video games are disputed, so you may want to add a "Back to the Future: The Animated Series", "Back to the Future: The Ride" or "Telltale Video Game" sub-section to "Behind the Scenes", respectively, after the main section to talk about the information gleaned from it that has significance to the article.
Alternately, you may choose to put "(The canonicity of [title of source] is disputed)"after the sentence.