A fan fiction, or fanfiction, is a fan-made story whose theme is based on an already-existing work of fiction. Characters and themes in the fan fiction work are typically taken from the work on which they are based, however, the fan fiction is, as its name suggests, made by a fan of the original work. Fan fictions may also include original characters and settings.
Fan fictions are protected by fair use guidelines so long as the fan fiction author does not try to claim credit for characters or settings he or she did not create.
On Federation Legacy Wiki, all fan fiction works must be based on Star Trek in some way. Crossover stories are allowed (i.e., any story that is based on Star Trek and another fandom, such as Doctor Who or Star Wars).