It's hard to give an all-encompassing view of Satan and his "followers" in just a quick reply to a forum board. But I guess this a great alternative to discussion groups for people with geography against them.
To begin with, evil is not an entity as some would understand an entity. This is the same as saying cold is not an entity; it is simply the absence of heat (think about it - there is no way to measure cold, we only can measure heat). Evil (sin) is simply the absence of good (God). That is also why nobody, not even God, could create evil - it is not something that can be create, destroyed, or even measured. Evil simply comes into "existence" when good (God) is taken out of the picture. That is precisely how Satan got his start as such as "evil" being. What Lucifer did was try to become God's equal; he did not try to be "better" than God or even completely de-thrown Him, he simply wanted to be equal with God. In order to do that, he relied on His own strength, he turned to himself and willingly forsook the power that God had given him (which was conciderable).
God saw what had happened with Lucifer turning against Him and had no other choice than to create a new home for Lucifer and the other angels he had rallied together for his cause. This is where we get Hell - the all-encompassing absence of God.
Soon after (maybe instantly) Lucifer realized his fate; that he could never overcome or be equal to God and that his final sentence is banishment and utter destruction. Instead of repenting, Lucifer got pissed, perhaps depressed, and decided to make it his mission to sway as many people away from God and eternal, blissful life as possible. In other words; he knows he's going down and he's going to bring as many people down with him as he can. From then on he started to take on new names and titles: the accuser, the tempter, the evil one, the devil, Satan, etc...
Worshipping Satan is really another form of humanist self worship, astrology and worshipping the stars, the Earth, or anything else that was created. Satan himself is a part of creation, he is not self-sustaining as only God Himself is. Satanism (or worshipping Satan) is really settling for second best. Instead of worshipping the Creator and relying on Him for strength and the promise of salvation, Satanists worship the created and only a limited source of strength and no promise, in fact no possibility, of salvation.
It's pretty short when we think about all the Satanism intales, but I guess it's a start.