Fan fiction, and its siblings fan art and doujinshi, consists of works written and/or drawn by fans of a particular story and using that story's characters and settings. In the world of anime and manga, doujinshi refers to fan-created manga based on a professionally-published manga. In fact, some manga creators, such as the female studio CLAMP, started out producing doujinshi.
Fan fiction provides young people with practice in creative writing, peer reviewing, and audience awareness in ways that their teachers may not be at all aware of.
For more information on fan fiction as an alternate form of literacy, see:
• "Adolescents' anime-inspired "fanfictions": an exploration of multiliteracies the authors explore "fanfiction" as a valid literacy practice in the context of the multiliteracies framework." Journal article by Kelly Chandler-Olcott, Donna Mahar; Journal of Adolescent & Adult Literacy, Vol. 46,