Sauron (literally "Abhorred"), named also Gorthaur, was the Maia of Aule at one time. But centuries ago he was deceived by Morgoth - the Lord of Darkness and Sauron became his the most loyal upholder. With the passage of years, he resembled his master more and more. He could change the form and his oratory skills mislead not a few. After Morgoth's defeat, Sauron even wanted to change his behaviour, but haughtiness and aversion to humble himself did not allow him to do this. He prefered to settle down in Middle-earth, in Mordor, which he fortified and where he built the mighty tower of Barad-dur. Sauron waged many wars with elves and men, brought about the downfall of Numenor, created the One Ring and these are not all of his ignoble deeds. After the lost battle and taking the One by Isildur, he lost his physical body and wandered around Middle-earth until he regained his former power. Sauron returned to Mordor, rebuilt the Barad-dur and was seen as the Red Eye. Hardly anybody could meet the eye of Sauron, who attracted all evil of Middle-earth: orcs, trolls, warlike men's tribes etc. Sauron was defeated on 25 march 3019 III A, when the Ring was destroyed. His spirit never came back to Middle-earth.
Some events are more widely described on the pages History and Ring-bearers.