Uub is a child that was introduced in the World Tournament after Goku defeats Majin Buu. When Buu was killed, Goku wished that he could be reincarnated as a good person so that he could battle with him once again. Uub is the result of this wish, hence his name (Buu backwards is Uub). Goku decides to train Uub at the end of Z and GT begins with the completion of Uub's training. Uub grew up a young warrior/farmer boy. Uub got his particular powers because he was created from Buu. Uub later fuses with the original Majin Buu. This transformation doesn't affect his appearance much, but increases his power to the point of almost matching Bebi in power. He then later fights Super #17 and then later the evil dragons.

(Source: Wikipedia)