Short answers, pulled from the story.
Sauron is the title character and main antagonist in J. R. R. Tolkien's The Lord of the Rings. He rules the land of Mordor and seeks to dominate all of Middle-earth using the power of the One Ring, which he lost and sought to recapture.
Sauron was one of the Maiar, lesser immortal spirits in Tolkien's mythology. He originally served Aulë, the smith of the Valar, but was drawn to Melkor's power and became Melkor's lieutenant and chief spy before rising to become the Dark Lord of the Third Age.
The Eye of Sauron is the image most often associated with him throughout The Lord of the Rings. Tolkien describes it as rimmed with fire, glazed and yellow as a cat's eye, with a black slit pupil that opens on a pit described as "a window into nothing". Sauron forbade his name to be written or spoken, so his Orcs referred to him only as "the Eye".
Sauron was destroyed when the creature Gollum seized the One Ring and fell into the Cracks of Doom inside Mount Doom, destroying both the Ring and himself. Because Sauron had put a great part of his own power into the Ring, its destruction ended him utterly.
In Peter Jackson's 2001-2003 film trilogy, Sauron was voiced by Alan Howard and portrayed in his brief humanoid form by Sala Baker. In Jackson's Hobbit adaptations, Sauron appears as the Necromancer and is voiced by Benedict Cumberbatch. In The Rings of Power on Amazon, he is played by Charlie Vickers.
Scholars have compared Sauron to Count Dracula from Bram Stoker's 1897 novel, noting both are parasitical, linked to darkness, and exert psychological domination through hypnotic eyes. Edward Lense also identified Balor of the Evil Eye from Celtic mythology as a parallel, a supernatural leader whose gaze could overcome an entire army and who ruled the dead from a tower of glass.