Well, there are obviously many things. It takes place in a different world, tends to have lower technology, has new and mysterious cultures and creatures...
Arguably, though, there is one thing that comes to mind immediately when people hear the word fantasy: MAGIC.
Magic sure is fun, isn't it? Adding it into a story can bring mystery and mysticism to the mundane, uniqueness to things that could otherwise be ordinary.
Magic also happens to be everywhere.
Every fantasy world seems to have a form of magic. From Middle Earth to the Nevernever to Narnia to the ever expanding Cosmere, Magic Systems are in just about everything fantasy.
So what sets each of these apart? What makes the magic in Harry Potter different from the magic in Mistborn?
Well, it's all about spectrum, really.
That's right. Time to get to some magical definitions.