Fan ficton(n.) –
fiction written by a fan of, and featuring characters from, a particular TV series, movie, etc.
Those who’ve never written fan fiction or had the desire won’t understand it, but I’ll try to explain. Fan fiction is, just as the definition says, a genre of writing. This isn’t to say that it’s better than any others or any worse. It just is. The difference between fan fiction and any other kind of fiction is that both the writer and the reader know up front that the characters and/or the setting aren’t of the author’s own creation. They’re borrowing it.