A Saiyan is a fighter from the planet Vegeta that was blown up by Frieza long ago because Frieza was afraid of the Saiyans. Very few Saiyans survived, among them were Goku, Radditz, Vegeta, and Broly. Goku as well we all know, was sent here to destroy Earth, but later changed his mind after a bump on the head. Radditz is his evil big brother. Vegeta is a cocky Saiyan Prince and Broly is a legendary Super Saiyan that was last seen 1,000 years ago. All Saiyans are born with black hair. Their hair does not grow from the time of birth. They all have a brown tail from just below their back, and have tremendous power. A true Saiyan can be very strong, and if he loses a number of battles, all without dying, he becomes that much more powerful.
The Oozaru form of a Saiyan is the large monkey form. This form of a Saiyan can only be achieved if the Saiyan still has their tail intact, and they are exposed to a full moon. The Saiyan then transforms into a Oozaru. The Oozaru is a large monkey with a power that is far greater power then the normal Saiyans power. In fact, if you take the power level of the Saiyan at the time of transformation, and multiply it by ten, your result is the power level of the Oozaru form of that Saiyan. The Oozaru is a huge animal that is virtually unstoppable due to its unbelievable power. Very few Saiyans ever go into this form; those included are Goku, Gohan, Vegeta, and Baby
A Super Saiyan is an advance form of a Saiyan, with tremendous power. When the transformation from Saiyan to Super Saiyan takes place for the first time, the Saiyan experiences great pain, and an tremendous explosion of power and ability. After their first transformation as they become Super Saiyan, it becomes more natural, and is not as painful. When the Saiyan becomes Super Saiyan, not only is his power increased, but his speed and special moves in ability as well. The most noticeable change in a Saiyan is the bright yellow hair that replaces that Saiyans natural hair color all throughout the body. With so many changes, a Super Saiyan is quite noticeable by its enemys, as if the golden glow around their body is not. As the Super Saiyan grows stronger, their muscles will bulge more, and can increase their power up to two times stronger. After enough strength training, and enough anger, they will reach the next level of Super Saiyan.
Super Saiyan 2
As if the Super Saiyan level was not advanced enough, the Saiyan can still extend his or her power more and more as they progress in the Super Saiyan levels. This level of a Saiyan is far more powerful then the Super Saiyan, and has grown longer hair. The hair is not much longer, but its a noticable differance. The muscles are not as bulged out as the full power of a Super Saiyan, and this enables the fighter to be faster, and stronger when necessary. The known fighters to reach this advanced level of Super Saiyan are Gohan, Vegeta, Goku, and Vegetto.
Super Saiyan 3
Once again, the Super Saiyan gets stronger. The Saiyans strength increases, but not as drastically as the change from Super Saiyan to Super Saiyan 2. This time there are some big noticeable changes. In this highly advanced form the hair grows much longer, down to almost the knees, and the hiar on their eyebrows totally disappears. The weirdest part about this form is that it is considered the last true form a Super Saiyan can get, but is still not the strongest(more on that later). Not too many Saiyans have gotten this far in strrength, and it is extremely difficult to achieve, involving a tremendous amount of power, and even more anger! The few Saiyans to have reached this form are Goku and Gotenks.
Super Saiyan 4
This form is not considered a Saiyan, more along the lines of a Saiyan and an Oozuru hybrid. Their strength increases even more, and is virtually immeasurable. In this form , the Saiyans normal hair color is returned, and it grows longer then its normal length, just between that of a Super Saiyan 2 and 3 hair length. This form is very cruel, and expreeses no fellings, (that's a big change for Goku!). This highly advanced form of a Saiyan does not wear a shirt, and his chest is puffed out quite far to boast this muscles. The entire body is covered with a dark red fur that is quite thick. In order for a Saiyan to reach this stage, he MUST have his original tail still intact, and in the Dragon Ball GT series, they are able to make their tail grow back using a special device Bulma makes called the Brute Ray. Goku and vegeta both reach this highly advanced and final stage of Super Saiyan.