Fanfiction

Fanfiction is fiction written by fans about a world and or universe created by a TV series, Movie, Book, Comic, Video Game and so forth. Basically, if you didn't create it, but you're writing about it, and it isn't recognized as canon, it is fanfiction.

A fanfiction story is called a fanfic or fic for short.

Types of fanfiction
Fanfiction comes in a wide assortment of flavors. Two main packages that these flavors come in are goodfic and badfic.

These flavors include:
 * Alternate Universe
 * Crossover
 * MST

External link
Wikipedia article on Fanfiction a wiki on the history of fanfiction