Sauron (which means "terrible" in Kvenian) is a supreme being, ainur, created by the thought of Iluvatar (i.e. "father of everything") from Flame Imperishable. He is one of the makers of Arda, a visible world, as well as a maiar. Sauron served Aule the Smith, the Lord of all the beings. He is the embodiment of Evil in "The Lord of the Rings".
"The Silmarillion" as well as "The Lord of the Rings" give a detailed account of Sauron's life. It is known that Sauron ruled Angband - a fortress, which stood on the way to the stronghold of Melkor. It was Melkor who corrupted Sauron and won him over to his dark side.
Sauron's foul deeds began in 457 of the First Age. At that time he invaded an isle of Tol Sirion and there created a harbour of Evil, and ruled over the werewolves. He could easily shift his shape to those of a wolf and other beings. In "The Silmarillion" there is a legend about Berenand Luthien Tinuviel, mentioning the name of Sauron. According to this he captured Beren, and killed Finrod Felagun, an elven Prince. But an elven maiden, Luthien, with a werewolf Huan came to rescue her beloved. Sauron took a form of the wolf but lost the fight. Then he took a form of a gigantic bat and hid himself in the woods of Taur-Nufuin till the beginning of the Great Battle. Indeed, Evil can await for its hour as Sauron does it. Melkor (or Morgoth) lost the battle and Sauron could do nothing but surrender. He was supposed to be sent to Aman for a trial. But pride and willfulness prevailed. Sauron preferred another escape to shame and humiliation.
Thus, Sauron gains his strengths anew. In about 1000 (Second Age) Sauron settled down in Mordor (which means "black earth" in Sindarin; there is, however, a similarity with the word "murder"). There he started building his mighty Barad-dur (which means "Dark Tower" in Sindarin), a fortress where he dwelt for many years. When building the Tower, he made use of his knowledge and skills learnt from Aule. Later he wormed himself into the Nordors' confidence (an elven tribe). Himself he named Annatar (i.e. "the Lord of Gifts" in Sindarin) and took a friendly look. But warm relations did not last long.
Sauron, cunning and crafty, planned with the help of the Nordors to forge the Rings of Power. But wise elves unraveled Sauron's sinister plans and a war (1693 - 1700 Second Age) broke out. However, Sauron managed to built his citadel and there, in the fire-mountain of Orodruin, forged the One Ring to control all the other Rings.
Because of his foul deeds he is called many unpleasant names. Many are his names in different Ages - Enemy, Gorthaur (i.e. "spreading horror" in Sindarin), the Great Eye, Black One and, of course, the Lord of the Rings. A scarlet pupil, or an eye without eyelashes became Sauron's emblem, which the orcs drew on their black bucklers.
If it hadn't been for Numenor's mighty fleet, as well as the dwarves of Moria and the elves of Lorien, Sauron would have won over the allied army. So, Sauron had to flee again and hide himself in Mordor.
Time passed and this time people drew Sauron's attention. He gave them nine Rings. Thus Nazguls, mighty shadows and Sauron's main servants, appeared. But people became jealous. In 3261 (Second Age) Ar-Pharazon, King of Numenor was thirsty of power, and demanded that Sauron submit to his authority and, seeing that the might of Numenoreans far outstripped his own, Sauron agreed. Thus, Sauron lost again, although he didn't fully surrender. It was in Sauron's power to influence people. Having told sweet lies to the Numenorean King, he convinced him to invade Aman. But the Rulers of the West united and sank Numenorean land. Sauron lost his flesh and thus couldn't influence people by changing his flesh any more.
In 3320 (Second Age) Sauron moved to Mordor. There he accumulated his dark powers again. In 3429 Sauron declared war on Gondor. The Last Alliance of people and elves resisted the Dark Lord. The Army reached Orodruin and there, on its slopes, destroyed the Enemy. Isildur, the eldest son of the King of Gondor, cut Sauron's finger off together with the Ring of Power. It seemed, that this would finally destroy the Lord of the Rings' might. But, no...
After the victory of the Last Alliance Mordor was closely supervised and Sauron couldn't pass the guards unseen. He hid himself in the almost impassable woods of the West. Sauron was a mighty sorcerer and his powers slowly returned to him. Then, he moved to dwell in the fortress of Dol Guldur (i.e. "bewitching hill" in Sindarin) and named himself the Necromancer. Sauron managed to conceal his true self from the White forces. Only in T.A. 2850 did Gandalf manage to prove that the Necromancer of Dol Guldur was indeed Sauron returned.
In 2941 the White Council drove the Necromancer out of his dwelling place. Sauron fled to Mordor again. It acted in his favour that after the Great Plague of T.A. 1636 the people of Gondor didn't keep an eye on the Dark Land any more.
The Dark Lord realized that the One Ring hadn't been destroyed for it nourished him with its power. He began to look for it everywhere and faithful nazguls helped him in the search. Luck was on Sauron's side. He caught Gollum who under tortures gave the names of the Shire and Baggins.
The Black Riders were not the only ones who helped the Dark Lord. A great number of spies, a strong army as well as Sauron, enslaved by the Palantir, were on Sauron's side.
On March 25, T.A. 3019 the Ring of Power fell into the fire of Orodruin. The Watchful Eye lost its might for ever. But no one said that the Enemy, or Evil which he personified, completely disappeared. Sauron again lost his ability to become visible. And nothing was left to suck energy from.
Undoubtedly, Sauron was the embodiment of Evil in Middle-earth. He forged the Ring of Power, invented the Dark Language, made new creatures. Also he was a cunning diplomat able to influence and negotiate with different people.
As soon as Sauron scents Evil, he will return again in order to take revenge on his offenders, set new rules and strive for absolute power.