Sauron is the evil Maia who becomes the Dark Lord in J.R.R. Tolkien’s legendarium.
Originally named Mairon, Sauron was a powerful spirit who served Morgoth, the first Dark Lord. After Morgoth’s defeat at the end of the First Age, Sauron became the principal enemy of the peoples of Middle-earth in the Second and Third Ages.
Sauron forged the One Ring in the fires of Mount Doom to control the other Rings of Power and dominate their bearers. His power became bound to the Ring, which is why its destruction is essential to defeating him in The Lord of the Rings.
Sauron is distinct from Morgoth, his former master, and from Saruman, a wizard who falls under his influence. The Witch-king is also a subordinate servant, not another name for Sauron.