Fanfiction is the writing of stories that are non-canon within a franchise by its fans. Fanfiction comes under many categories and types, varying from shipping fanfics focused on a particular ship, comedies which are aimed for simply laughs, continuations which continue the story, and some are focused on giving certain characters the spotlight for a story.
Fanfiction is decidedly non-canon, as it is not approved by official sources. However, that is not to say that a fanfic necessarily contravenes Canon itself, as some are written in so as to not only not contradict Canon, but could easily be slid between episodes or scenes of episodes without affecting any aspect of Canon.
The most popular fanfiction site is Fanfiction.net, which provides a common ground for fanfiction to be written and read.
Fanfiction and the Kim Possible Fanon Wiki:Edit
As a fanon wiki, the KP Fanon Wiki will host pages detailing and overviewing fanfiction and its contents, much as a wiki overviews any series or topic. However, it is not a site for the posting of fanfiction in as of itself.