Sauron was originally a Maia to Aule. (A maia is a lesser god who is subservient to the greater ones)
He eventually went to the service of Morgoth, and became his lieutenant.
Sauron returned and entered Mordor and erected the tower of Barad-Dur. When the Numenoreans challenged him, he went as a prisoner, and eventually became a trusted advisor. The Numenoreans tried to fight the greater gods and they all died.
Once he came back, he slipped back to his old ways. He aided the Elves in making rings of
power, and eventually he made the One Ring, or the Ruling Ring.
Later on, Sauron gave some of the rings of
power to
men, and eventually they fell under the
power of the Ring and became the Nazgul.
The words on the ring are: One ring to bring them all and in the darkness bind them.
The elves and
men formed the "last alliance" and went to fight the hosts of Mordor. They laid siege to Mordor. Then Sauron came and killed quite a few elves and
men before killing Gil-Galad and Elendil before Isildur
cut the Ring from his
finger. Barad-Dur was leveled to the ground, but its foundation was not destroyed, because its
power was tied with the ruling Ring.
Sauron hid in Mirkwood, and the wizards came from over the sea to challenge Sauron's
power. (The wizards are also lesser gods) Sauron eventually went back to Mordor and rebuilt Barad-Dur.
Eventually, Frodo and Sam threw the Ring into Orodruin, or Mount Doom, and the Ring was destroyed and the
power of Sauron was diminished forever.
