There has been a popular misconception, basically because of the false information provided by the media, that Satanism involves things like human sacrifice, drug abuse, self mutilation, desecration of churches etc. This is bullshit. While such activities do take place, they are practiced by teen kiddies who have severe mental problems. Satanism is more like a philosophy.
The Satanist faith was formed Anton Szandor LaVey in 1966. He also wrote the book "Satanic Bible" to outline the basic tenets of their beliefs. Basically, they believe ego gratification to be paramount of all human activities. They despise charity towards the weak and embrace the strong, and have very little to do with the guy with horns and cloven hoofs. The reason behind the title "Satanism" is that Mr. LeVey thought it sounded like catchy title for his new religion.
The Satanic Bible itself is diveded into two sections. The first dealing with philosophy, and the second dealing with Satanic magic, rituals, ceremonies etc. The second part is probably the main reason the Satanists have been so misunderstood. Yes, it has stuff like "Invocation employed towards the conjuration of destruction", which mentions symbolic human sacrifice. While this might sound gruesome, it's still only symbolic. Actully, it means masturbation during the ceremony. LeVey specifically states that killing humans is a criminal act.
Satan worshippers are a totally different matter. They are the ones who commit the crimes. Satan worshippers are typically arranged in gangs (I'm sure they call them "Covens", since it sounds cooler than "Gang"), usually led by a small-time criminal who came up with the idea in order to meet young women and to form a posse to perform crimes for him. These kids then meet in dark forests and graveyards, mutilate themselves, rape each other and take heavy drugs.
According to the interviews I have seen, most Satan worshippers have had some severe traumas in their childhood. Some were teased in school because they were different, some were sexually abused, some had lost a parent. Their insecurity was then dissipated by promises of "Satanic power" by someone.
Comparing Satanism with Satan worship is like comparing Christianity with the Inquisition. While they have some things in common, the other is merely a perverted version of the idea.