Sauron, sometimes called the Dark Lord or the Lord of the Rings, was a Maiar who was seduced by Melkor Morgoth early in the First Age and became the chief of his servants. At the end of the First Age Sauron was given a chance to change for the better, but would submit himself to no such humiliation, and thus he fled deep into Middle-earth.
In the Second Age Sauron had much power, and soon he established his kingdom in Mordor and began the construction of Barad-dûr. During the Dark Second Age Sauron corrupted many of the Free Folk to his own evil ways. Before long, Sauron, having learned the art of Ring-Making from Aule, whom he served as a Maiar before his corruption by Melkor, made the One Ring, pouring much of his power into it. It was with this One Ring that he was able to complete Barad-dûr. Soon, in the War of Sauron and Elves Sauron overran Eregion and destroyed Eriador.
Sauron eventually was captured and taken to Númenor, but here he corrupted the Dúnedain and turned the island into a place of worship in honor of Morgoth. Sauron told the then King of Númenor to claim immortality by invading Valinor, but to the shock of Sauron the Valar called upon Ilúvatar, and Númenor was destroyed and Sauron was destroyed and allowed never again to take a fair form with which he could seduce Men.
Sauron soon returned to Mordor to muster his forces, and there he led many attacks upon the Free Peoples of Middle-earth. During this time he claimed Minas Ithil. This went on until he was defeated upon Dagorlad, the Battle Plain, and he quickly retreated to Mordor. When the end of the Second Age was drawing nigh the Last Alliance of Men and Elves attack Barad-dûr and upon the slopes of Orodruin the final battle of the Second Age was fought. Gil-galad and Elendil, father of Isildur, fought against Sauron, but both were slain before Isildur seized the Ring and killed Sauron.
In the Third Age Sauron used all of his energy to search for the Ring. For much of this time he dwelt in Dol Guldur, and he hid his identity by calling himself the Necromancer. Once he again had physical form he began to slowly ruin the Kingdoms of the Dúnedain. He sent the Lord of His Ringwraiths to the north to found Angmar, the Witch Realm. In 2941, after repeated investigations by the White Council, Sauron was forced to flee from Dol Guldur, but he found Mordor made ready for him by the Ringwraiths, and found out that they had stolen for him a Seeing-stone. He had soon rebuilt Barad-dûr and was gathering his power there.
Gandalf and Elrond realized that the only way to forever stop Sauron would be to destroy his Ring, for he had put so much of his physical and mental energy into it that if it was unmade he would have no energy to regain physical form. Thus it was that Frodo volunteered himself to bring the Ring to Orodruin. This was done and Sauron and his Ringwraiths were destroyed forever in 3019 of the Third Age. In the Third Age Sauron usually appeared as an Eye.
Thanks to Khaos1976 for some information!