Vegeta is a powerful Saiyan. He is a "Super Elitist" and a prince, the son of King Vegeta, past ruler of the Saiyans. Vegeta and his comrade Nappa are two of the last surviving Saiyans. They come to Earth to destroy all life and sell the planet. Though physically smaller than Goku, Vegeta can power up to unbelievable levels destroying entire planets. He is a fierce rival of Goku's, striving always to seek power levels as high or higher than Goku and to reach Super Saiyan level. Ambitious and determined, Vegeta wants the Dragon Balls to gain immortality. He is determined to destroy Frieza. After the Namek Saga, he fights on the side of the good guys and becomes the father of Trunks.
(taken from www.dragonballz.com)