Originally the demon Svarbhanu, son of Viprachitti and Sinhika.
Vishnu beheaded him and he became Rahu and Ketu, the lunar nodes, two of the nine planets, Navagraha.
Rahu is the ascending lunar node, the head.
Ketu is the descending lunar node, the body.
Rahu and Ketu are responsible for the lunar and solar eclipses, which they devour on this event, as they once betrayed Svarbhanu.
Read about Rahu here
http://www.sushmajee.com/shani/1-kathaa-1-19/9-raahu-ketu.htm
http://utkarshspeak.blogspot.com/2010/10/mythical-explanation-of-eclipse-indian.html