What is the difference between wicca and satanism?

Satanism is a vastly differentiated group of ideological and philosophical beliefs that revolve around the veneration of Satan (a biblical invention) or similarly rebellious figures, that arrived in the late twentieth century but may be based on earlier manifestations in part. Some may be atheistic who base their philosophy on a symbol (rather than a figure) that represents the self-indulgent nature of humanity.

Atheism is simply the absence of a belief in gods and the supernatural.