Sauron is a Maia, one of the ancient wizard-gods of Middle Earth, who was corrupted by evil and became a cruel, merciless conqueror. He forged a ring of power, the One Ring, that gives vast reality-warping powers to whoever wields it, in order to use it as a powerful weapon. However, he lost that ring in a battle.
He decided to attack Middle Earth with an army of orcs. Gandalf was sent by Galadriel to defeat him, but Sauron easily defeated him and took Gandalf prisoner.
He was finally killed by Aragorn's father, but his ghost haunted Middle Earth in the form of a giant floating eyeball. He launched his attack on Middle Earth with the intent of conquering it. However, the Fellowship of the Ring killed his armies, and the One Ring was destroyed, defeating him once and for all.
