Refers to random humor partaining to a particular show/movie/book that often makes no sense to people outside the fandom. The thought behind the term is that one would have to be on crack to think up some of the jokes. Also extremely adictive (Ex: "crack breeds crack")
Can come in the form of fanfiction, fanart, photo edits, etc, etc.
The fan crack destroys my brain cells... in a happy way.