Let me get some common assumptions out of the way. Satanists do not sexually abuse anyone, especially not children. They do not sacrifice animals, nor do they drink blood or eat any human organs in the course of a ritual. They don't worship any god. In fact, they don't even worship Satan, or at least the Judeo-Christian concept of Satan.
Now, to get to the core of Satanism. Satanism is basically atheism with an emphasis on the self. It asserts that all people are, by nature, only animals and have the same animalistic tendencies. There are some basic "commandments" and "sins" but they all involve paying homage to the self (a.k.a. "Satan").
The reason that they use the word "Satan" is to imply (or in this case directly state) opposition. For years and years in many religions there has been a seperation between a diety that sets the rules which everyone must conform to (god) and another power that is against this rigid status quo (satan). The Satanists use this concept of satan in their belief system.
The Church of Satan has a general outline of their beliefs that I really haven't delved into much. They do have a list of "sins" etc. Check it out, it's worthwhile!
Here are some links that sould explain the subject much better than I ever could.
And here's one to laugh over:
And, as always, email me!
Oh no! The dangers of Satanism!!!