There are a couple of hundred definitions out there on what satanism really is.
Here's what I think satanism is about - based on Anton LaVey's Satanic Bible.

In 1967, Anton Szandor LaVey created the Church of Satan (and not on Walpurgisnight 1966 as he liked to claim).

Q: Is Satanism a religion?

A: Satanism is not a religion, it's an UNreligion.
Satanists do not believe in the existance of Gods, The Divinity
of the Bible, Angels, Heaven or Hell, The Devil, Satan, Spirits, Demons,...

Satanists do not hold belief in supernatural beings. Period.

But then again, it depends on your definition of 'religion', I guess. If religion is a synonym of 'belief', satanism is a religion.

Q: Do Satanist worship Satan?

A: Satanists are not 'Devil' worshippers (The Greek word 'diabollus' means 'great deceiver', everything a Satanist opposes to). Satan comes from Hebrew 'an opposer'.

The Roman god Lucifer (from Latin lux=light, and ferre=to bring) was the bearer of light, the personification of enlightenment. In Christian mythology he became synonymous with evil, the fallen angel. In the Bible (Isaiah 14:12-15), the King of Babylon is referred to as Lucifer. Satanists refer to Lucifer in the original meaning of the word.

The being Satan is the scapegoat / invention of Christians and monotheistic dualistic
theories, and not a real being.

The belief that "Satan, The Dark Lord" (and therefor a real
entity) is the strongest of two (good and evil) forces, or the evil component of a cosmic
dualistic battle, is devil-worshipping, and has got nothing in common with satanism.

During the late Middle Ages, Catholic Church taught that some "Witches," mostly women, worshipped Satan, killed babies, flew through the air on broomsticks,... Satanism as it is most commonly referred to, is an invention of the Catholic Church.

--> Satanism is a human potential movement, based on real objects, real things, real events in what they perceive to be the real world. Satanists recognize themselves as gods, and they hold their own perspective on life as holy and revere their own experiences as the only truth we can ever know.