Fan fiction involves original stories about characters from mainstream commercial franchises written by fans. Often these take the premises of the original commercial franchise and bend them in startling new directions. For instance, there are thousands of examples of fiction written by fans of Harry Potter who want more or different stories about the characters and world created by J. K. Rowling.
Much of fan fiction is not of very high quality, but there are lots of well written fan fictions; some are very popular with other fans of the official franchise. As with fanvids, fan art, and other fan appropriation genres, the results are often more inclusive or at least less mainstream than the plots and characters in the official media.
Fan fiction goes back at least to the 1960s, when fans began writing new stories about the characters and world of Star Trek and other franchises.