Many people have misconceptions about who Satan is, or at least who Christians believe he is. For some reason, possibly due to the animistic religions of Europe, and then some in Asia and the Americas and in Africa, this notion of Satan as a sort of anti-God has arisen. This is by all means not true. I guess to understand the Christian view of Satan, we must also understand what evil is.
In Christianity, God and good are one in the same. Evil is simply the absence of good, the absence of God. A good model for understanding would be the relationship between heat and cold. Cold is really the absence of heat. Evil is by and large a complete absence of good, that is, God is not present. So, to oppose the will of God, that is, to do something God will have no part of, is rendered an evil deed, a sin. This idea of God being all that is good is not limited to the Christian scope, but is found in almost anyone's idea of God.
That said, if evil at its base form is nothing but the lack of good, then Satan has no real power. He is not a 'bad god'. And furthermore, God did not 'create' evil, as some people claim. Evil is more of a vacuum, a place or state devoid of God. Earliest mentions of Satan refer to him as The Adversary, a servant of God who tested man's love of God and his will to do God's work. He did anything from tricking humans to outright hurting them (Job). He, Lucifer, was considered one of Gods finest creations. That is, until he opposed God's will.
Satan/Lucifer as we all know, was once an angel. He basically got uppity and decided that he was powerful enough to question and oppose God. His damnation to Hell was his own doing, not God's retaliation. By separating himself from God, he separated himself from all that is good and beautiful. Satan is not the monarch of the underorld, as many people see him, but a soul who is just as tortured and pathetic as any other in Hell. His only power is to lie, to deceive, bluntly and simplistically put, he is the little devil on your shoulder.
To say Satan represents free will is ignorant of all Christian beliefs. To say that man is a slave of God is also ignorant, in light of the Christian viewpoint (I'd say so for Judaism and Islam too, but I don't know enough...maybe someone can help me out). Satan is an angel. In Judeo-Christian 'mythology', there are a number of sentient beings created by God to love and adore Him. There are the cherubim (not the fat little babies, but majestic warriors), the seraphim (6-winged creatures whose true form no one knows), and all the other angels and ethereal beings. The last to be created was humankind. The Christian view on the reason God created humanity was so that he could have people who choose Him out of will and love rather than any constraint. An angel literally cannot choose to serve or to love God. They ar emade to do it, and that's what they do. The minute one gets any ideas about not doing so, by nature he is immediately separated from God, from good, and has no chance to ever return. Satan has no free will. He had one chance and blew it, end of story. Human beings can constantly choose to reject or accept God, and will always be forgiven for their wrongdoings, no matter how bad they are, if only they seek it. True, humans will meet the same fate as Satan if they reject God, but its not God who sends people to Hell, but arther their own will. In fact, the belief is that God will do everything in his power to stop a soul from falling into Hell. His love is unconditional and it pains Him to see someone leave Him. The fact though, is that humans can, at any time, choose God or reject Him. Humans, and humans alone, have this freedom.
The question still remaining is what does Satan do? Satan, embittered by his fall, can feel only hate. He hates God for creating Him, and he hates everything associated with God, particularly mankind, who have the ability to choose God freely. So, he attempts to turn people against God, because this hurts Him. It's the only thing Satan can hope to do against God, and that's ehat he does. The temptation of Satan in the Christian viewpoint can be seen as a man luring someone else over a cliff with false promises. But the truth is that man can at any time, choose God or reject Him. Hell is not seen as punishment for bad deeds, but a self-brought separation from God and all that is good, meaning absolute suffering and darkness.
Once again, I am only showing a particular belief about complex ideas, the one I believe, but I will not say that this is absolute. Everyone has their own beliefs, and although I believe one way does not mean its wrong for other people to believe differently. I am merely expressing my viewpoint, and the viewpoint of Christianity on this issue, which is often misunderstood at best.