I noticed most of the other definitions here completely trash fanfiction.
First, definition: Fanfiction is when someone takes either the story or characters (or both) of a certain piece of work, whether it be a novel, tv show, movie, etc, and create their own story based on it. Sometimes people will take characters from one movie and put them in another, which is called a cross-over
Most people who bash fanfiction are not willing to look past the fact that it's based on something else to see that it could be worth reading IF you like the subject. If they don't like the subject, then obviously they are going to automatically dismiss the fanfiction.
It is true, however, that some fanfictions are rather poorly written and only a few hundred words, and it is also true that some people just write them so they can have their favorite characters have sex (lemon
). But, if you take the time to find something decent, you can end up with a fanfiction story that is so close to the original piece of art, that you'd barely notice the difference.
Mary decided to write a fanfiction based on a story she had read. She liked the idea of two of the characters as a couple, but it wasn't in the original plot, so she twisted the story around a bit and evolved the characters so that it seemed as though they actually *could* have been together in the first place. (Good example, it's rare that someone does it perfectly but there are some out there)
Sue was so angry that her two favorite characters never got together that she wrote her own story in which the first thing they did was have sex and get married. (Bad example, but sadly sometimes true)