For example, if I enjoy the Harry Potter book series by J.K. Rowling, but wish Book 7 had ended a little differently, and I write my own ending, I have just created fan fiction.

To take it one step further, fan fic writers (also called fictors) often post their fan fiction online for other fans to read and review. The most popular fan fiction site is appropriate named FanFiction.net, but there are lots of private web sites devoted to fan fiction based around a particular work.

Some common fan fiction themes are rearranging relationships (For example, writing a story in which Harry and Hermione are a couple instead of Ron and Hermione), cross-overs (Suddenly Gandalf shows up and starts dueling with Dumbledore), and completely way-out-there rewrites (Harry Potter takes place in the future as a scifi thriller with science instead of magic). The possibilities are endless, really.

For a full glossary of fan fiction terms (fictor, canon, ship, etc.) you can visit this site.

CAUTION: Fan fiction can be fun and/or funny, but there are also varieties of fan fiction that just plain suck, or may not be suitable for all audiences (graphic violence, sex, etc.). Be careful out there!