Satan (aka the Devil) is the main adversary of God in Christianity. There are mountains of stories and conflicting opinions about the particulars of this evil deity. A widely accepted story is that he was once a chief angel of God, but that he became dissatisfied and led a rebellion against the big guy. After Satan was defeated, he and the other rebel angels were cast out of Heaven.
Now, Satan spends most of his time tempting people away from God and causing as much damage in the world as he possibly can. When the final battle of Armageddon comes, though, Satan will be defeated and cast into the fiery pit of Hell along with all the souls he's stolen from God. Satan is also the opponent of God in Judaism as well as Islam, where he's known as Shaitan.