Originally there were two races on the planet Vegeta, the Saiyans (can also be spelt as `Saiyajin') and the Tuffles (known as Tsufurujin in the original). This planet was destroyed by Frieza, and all the Saiyans were killed except for Prince Vegeta who was kept in Frieza's employ, and Goku who was sent to Earth as a child.
Saiyans look very similar to Humanss, except that they are born with furry monkey tails, are incredibly strong, and have the ability to transform. They can turn into huge ape-like creatures if they have a tail and are exposed to moonlight. They can also turn into Super Saiyans.
Saiyans have the ability to interbreed successfully with Humans, meaning that their genetic material must be similar, or an unconventional conception takes place. The resulting offspring are known as Halfbreed Saiyans.