Goku, the Saiyan son of Bardock, was sent to Earth to destroy it at a very young age. When he was just a boy he hit his head forgetting about his misssion. His "grandfather" Gohan raised him and taught him martial arts. When Goku grew up he became the hero of the universe defending it whenever needed. Goku and his wife, Chi-Chi, had two sons. The first being Gohan named after Goku's "grandfather", and the second is Goten who has not yet been introduced in DBZ.