Saiyan- This is the regular form of a saiyan, the saiyan has black hair.
Oozaru- When a saiyan is exposed to the Full moon's light they will turn into a huge ape like creature. The saiyans power is increased by a lot but the Oozauru stage can be ended when the Saiyans tail is cut off when he is Oozaru or to get rid of the moonlight. Advantage: Makes saiyan huge and very dangerous Disadvantage: Saiyan will attack anything and anybody
Super Saiyan-This is a form of a Saiyan where he is very powerful, and his hair turns gold. The only way to obtain this level is to be strong enough, and to be totally enraged over something. Other physical changes are that his eyes turn turqoise, his hair sticks up, and an increase in muscle mass. This is a very powerful stage. Half-blood Saiyans are able to reach this level also.Goku was the first to reach it though. Advantage: All attributess increase Disadvantage: Takes a lot of energy during the transformation
Ultra Super Saiyan- This stage is in between SSJ and SSJ2 but you do not have to achieve it to be SSJ2. When a saiyan turns USSJ his hair is more spikey and has more gold in the hair, the saiyans body size increases greatly as well as their power but they lack speed due to that. That is displayed when trunks transforms against Cell but his lack of speed made his great power useless. Advantage: Huge strength increase Disadvantage: Bigger body reduces speed greatly
Super Saiyan 2-This is the same as above only the hair sticks up more, and the person, is able to control his speed, power, etc. but when you chose to use more of one, you have to lower the other two. This stage isn't really that important, but Gohan was the first to reach it. Advantage: All attributes can be adjusted to saiyans liking Disadvantage: None
Super Saiyan 3-In this stage the hair is gold, and the eyes are turqoise, but the hair is very long, thick, and dense. It grows to about the waist, and the eyebrows disappear to give them a meaner look. Also this stage you use alot of energy, so you can only stay in it for a short time. This is more powerful than stages 1, and 2. Goku was the first to reach it. Advantage: All attributes increase greatly Disadvantage: Takes a lot of energy for this transformation
Gold Oozaru- Much more powerful than regular oozaru. This stage is achieved when a saiyan looks at the Full Earth from the planet Plant. Physically the gold oozaru has Gold fur around his body. Advantage: Size and strength increase Disadvantage: Same as oozaru
Super Saiyan 4-This stage is completely different than the first three, the hair is no longer gold, its long, and black, and red fur covers the body. This too uses alot of power so you cant use it for that long, but the overal power and speed greatley exceeds the first three. Many people say that this isn't a true stage of Super Saiyan, because the hair isn't gold. Goku was the first to reach this goal also.  Advantage: All attributes increase Disadvantage: Very hard to achieve this level