There seems to be confusion about the terms Wicca, Theistic Satanism and Atheistic Satanism. Personally I'm not into any of these, but I would bet someone here is...
Can anyone explain it?
Modern Wiccaism is a religion based on worship if nature. In this instance, the word nature is sort of vague. The confusion began when the word "witch" was applied to Satan worshippers. It's possible some Wiccans in the Middle Ages crossed over into Satanic circles and brought some Wiccan traditions with them. There is a tremendous amount of Satanic worship lore, which borrows freely from Pagan rituals which predate Christianity.
Theistic Satanism is a cult(for lack of a better word) which recognises the power of God and His battle with Satan, for the souls of the world. In this case, the Theistic Satanist is on Satan's side.
Atheistic Satanism is a contradiction in terms. The most it could ever be is a costume party, with fun games.