Annatar was the fair form that Sauron assumed in order to persuade the Free Peoples into aiding him. He used it several times and it resembled him when he was Mairon.
Sauron assumed this form when he gave the Rings of Power to the kings of Men, Elves and Dwarves in the Second Age. He later used it when he was taken "prisoner" by King Ar-Pharazon known also as Suladan, in order to convince him to make him and advisor to control the city of Numenor. Eru sent a wave that saved the fiar Numenoreans and killed the traitors, but also destroyed the Annatar form.