Gods are something people worship.
The question whether or not the god really exists is another thing. Under this discussion we might restrict gods to those beings who can offer special powers or other magical benefits to their followers. There are different kinds of gods based on their origin, nature, what source of power they use, etc. Some gods might not play by the same rules as the rest of the magic-users. Some gods use powers which are different from, or beyond, ordinary magic and can bestow extra-magical powers on their followers.