They come up with different storylines and expand some of the plots that are already in the other books.
And fan-fiction also gives minor characters a chance to be the most important ones for a change.
It's also a really good way of helping people improve their writing skills.
We all write a little bit of it, although it's tough to find the time at the moment because of school work.
Fan-fiction helps in our exams
So when the summer comes we'll be able to write loads more!
But writing fan-fiction has helped with our school work too, since it helps with out English exams and we can all type loads quicker now.
Everyone really likes the chance to get more involved in the world of Harry Potter."
Emma, Sam, Charlotte, Jenny and Katy, in a queue, London
Why don't you write us a Press Pack report - and get it published on the site?!
It can be about anything that's happened in your local area - or your views on the news.