Short for fan fiction, is not a plural. Usually based on books, brands, movies ect.

Often times, fanfic is a bunch of crap and takes away from the story's real plot. However, there are a few good pieces out there that don't stab and kill the real thing

Many authors allow fanfic to be posted without consent, as long as it mentions that the characters were created by them. However, other authors (AKA killjoys) are idiots and don't see the free promoting of the book, and don't alllow fanfic at all.