Short answers, pulled from the story.
Sauron began as a creature named Mairon who served Aulë, the smith of the Valar. He betrayed his masters to join Melkor and transformed into Gorthaur meaning The Cruel before taking the name Sauron.
Sauron assumed the fair appearance of Annatar Lord of Gifts to seduce Celebrimbor's Elven-smiths. He taught them arts and magic helping them forge the Rings of Power while secretly creating the One Ring to rule all others.
Isildur cutting the One Ring from Sauron's hand left him as a shapeless evil for a thousand years. He eventually reembodied as the Necromancer hiding at Dol Guldur in Mirkwood before returning openly to rebuild Barad-dûr.
The Eye serves as the image most often associated with Sauron because he did not allow his name to be written or spoken. Christopher Tolkien noted how his father identified the Eye with the mind and will of Sauron.
Alan Howard provided the voice for Sauron in the 2001, 2003 film trilogy directed by Peter Jackson. Sala Baker portrayed the character as a large humanoid figure clad in spiky black armour during brief appearances.