Saiyan
A Saiyan is a fighter from the Planet Vegeta that was blown up by Frieza a long time ago because Frieza was afraid of the Saiyans. Very few Saiyans survived, among them were Goku, Vegeta, Raditz, Nappa, and Broly. Goku, as we all know, was sent here to destroy Earth, but later changed his mind after a bump on the head. Raditz is his evil big brother. Vegeta is the Saiyan Prince and Nappa was his sidekick (sorta). Broly is a legendary Super Saiyan that was last seen 1,000 years ago. All Saiyan are born with black hair. Their hair does not grow from their 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 much more powerful.

Oozaru
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 an Oozaru. The Oozaru is a large monkey with a power that has far greater power than the normal Saiyan's 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 from that Saiyan. The Oozaru is a huge animal that is virtually unstoppable due to its unbelievable power.

Super Saiyan
A Super Saiyan is an advanced 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 a tremendous explosion of power and ability. After their first transformation as they become Super Saiyan, it begins to come more natural. and is not as painful. When the Saiyan becomes a Super Saiyan, not only is his power increased, but his speed and special moves in ability as well. The most noticable change in a Saiyan is the bright yellow hair that replaces the Saiyan's natural hair color all throughout the body. With so many changes, a Super Saiyan is quite noticable by its enemy, as if the golden glow that appears around their body is not. As the Super Saiyan grows stronger, their muscles will bulge more, and can increase in 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 itself 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 than a Super Saiyan, and has grown longer hair. The hair is not much longer, but it's a noticeable difference. The muscles are not as bulged out as the full power of a Super Saiyan, and this enables the fighter to be fast, and stronger when necessary. The known fighters to reach this advanced level of Super Saiyan are Gohan during the Cell battle, Vegeta when under the influence of Babidi, Goku when fighting Vegeta, when Goku and Vegeta fuse for the first time into Gogeta, and when Goku and Vegeta fuse the second time to form Vegetto.

Mystic Super Saiyan 2
This form of Super Saiyan was only mentioned once, during the Majin Buu series, It is supposedly unachievable, and Gohan does attempt at it, but does not succeed. No one knows what a Mystic Super Saiyan looks like, but it was rumored to have an eerie glow to him. On to the next stage . . .

Super Saiyan 3
Once again, the Saiyan gets stronger. The Saiyan's strength increases, but not as drastically as the change from Super Saiyan to Super Saiyan 2. This time there are some big noticable changes. In this highly advanced form the hair grows much longer, down to almost the kneews, and the hair 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 strength, 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 white battling Majin Buu, and Gotenks, the fused form of Goten and Trunks. Still the Saiyans can get stronger.

Super Saiyan 4
This form is not considered a Saiyan, more along the lines of a Saiyan and an Oozaru hybrid. Their strength increases even more, and is virtually immeasureable. In this form, the Saiyan's normal hair color is returned, and it grows longer than its normal length, just between that of the Super Saiyan 2 and 3 hair length. This form is very cruel and expresses no feelings (that's a big change for Goku!). This highly advanced form of Saiyan does not wear a shirt, and his chest is puffed out quite far to boast his muscles. Most parts of the body are 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 intact, and in Dragon Ball GT series, they are able to make their tails grow back using a special device Bulma makes called the Brute Ray. Goku and Vegeta both reach this highly advanced and final stage of the Super Saiyan.

Conclusion
The Saiyan race is very powerful and is almost impossible to stop. With every battle lost a Saiyan grows stronger; there fore, the Saiyans are invincible in strength. Until they are overwhelmed, they will just become stronger by transforming into a Super Saiyan.