The Annunaki were gods of ancient Sumer, one of the earliest known human civilizations. The head of the Anunnaki council was the Great Anu. His throne was inherited by Enlil, resulting in a dispute between Enlil and his cousin Enki regarding who was the rightful leader. Enki was an alchemist and was said to have generated mankind.
Mainstream science explains the Anunnaki (plural) as simply mythological beings created by the imagination of the ancient Sumerian religion. Some scholars such as Zecharia Sitchin and Laurence Gardner claim they were real, physical entities who literally came from outer space to Earth; spacemen who came to Earth in antiquity and pretty much created Homo sapiens 150,000 years ago.
According to Sitchin, these beings live on a planet called Nibiru, allegedly the 12th planet of our own solar system. Some believe they are of reptilian nature, and that they feed on human flesh.
Some of this theory is based on the idea that we have not found a "missing link" and also on the knowledge the Sumerians had about the solar system, which strikes some people as surprisingly advanced given Sumerian culture as a whole.
Sitchin asserts that the Sumerians credited all their knowledge to the Anunnaki rather than to their own doing.
See also: Nibiru (home of the Anunnaki)