Sauron was the Dark Lord Morgoth's abominable lieutenant during the First Age. He was also the creator of the Rings of Power and the Dark Lord of Mordor, whom the Fellowship of the Ring sought to defeat.
In the earliest of days, before the Valar entered Arda, Sauron was in origin an "angelic" spirit called a Maia. He was, at first, a powerful servant of Aulë, the Smith, a greater angelic spirit, one of the Valar, or ruling powers of the world. However, Sauron was soon subverted by the Dark Lord Melkor (later known as Morgoth, an evil Vala), and Sauron himself turned to evil. Ever after, Sauron served Morgoth faithfully, and even in later days, after Morgoth was defeated and locked outside the confines of the world, Sauron encouraged and coerced Men to worship Morgoth as god. However, while Morgoth wanted to either control or destroy the very matter of Arda itself, Sauron's desire was to dominate the minds and wills of its creatures.