Christians: What does Satan do?

I'm not a Christian, but I've studied his role in the religion at length. It fascinates me, because it's so weird. In the Old Testament, particularly in the Book of Job, he seems to have a role similar to a district attorney, accusing humans of violating God's law, and prosecuting them. That would tend to indicate that he's on the side of good. But then, in the New Testament, much is made of him rebelling against God, which supposedly took place before the events of Genesis. So apparently, he goes around tempting people to sin against God, even while he's supposed to be acting as God's prosecutor.