Similar to an RPG, each person takes the view of one or more of the characters, and writes the story from the point of view of that person. These stories involve much effort on the parts of all involved, but fans usually find it rewarding to read such works. Other fanfics leave the realm of the written entirely. Other films, or screenplays written by fans, also qualify. Some lucky few fans actually get their ideas used on the shows or in the books.
Fans draw or design pictures of the universe of their choice, but color it through their own eyes.
The enjoyment fans get from IEE's is amazing. People can take these worlds and watch them evolve. When they are dissatisfied with something, they can take matters into their own hands and change the IEE to suit their wants. People from all over are drawn together by a common interest in these worlds. And they can share with each other through the wonderful media of Fan Fics and Fan Art.