Fan art is when you paint characters or specific things that already exist. It can be anything from movies and manga to books and comic books.
I absolutely love doing fan art. You can do so much with pre-existing media and you can show your ideas to others who enjoy the same media. This is my favourite way of interacting with different groups within different media.
It is also a way to supplement something to e.g. a movie that you think is missing. In my case, I made a whole collection of wedding dresses into an animated series because I thought it was missing after 9 seasons