From Encyc

Sauron is a fictional character from the Tolkien books. He was one of the Maia, or lesser gods, who followed Melkor (later Morgoth). He fled to Middle Earth and enjoyed a long career in which he caused trouble on Numenor and then on the mainland.

Sauron (as Annatar) back when he could still appear fair.
Artist's depiction of Sauron

He invested a large amount of his power in the one ring, which worked out as long as he still had it but then Isildur cut off his finger and took the ring.

The one ring Sauron made to control all the others

He spent the Third Age searching for the ring while steadily regrowing his power and reestablishing control over Mordor and the surrounding area. His plan nearly succeeded but a hobbit brought the ring to Mount Doom, where it was destroyed.