From Diablo Wiki
Fanmade art is the most apparent and easiest form to appreciate. We are given a ready-made picture of Sanctuary and its inhabitants. It takes longer time to get in to fan fiction stories and it can take longer to create, but especially in the case of stories like The Key, it's well worth the effort.
Mixing these up we get fan classes and fan monsters. Both exciting ideas of what could exist in the world of Diablo.
Read more about fan creation, where to find more and how to publish your own work in the fan creation article.