A Saiyan is a Human-like creature. Saiyans have something that humans don't have which is a Tail. With these tails, they can transform into a giant ape like creature called Oozaru. Saiyans have a very strong spirit and will never stop until they win any battle they are in. Saiyans may look human but they are very powerful and will have no trouble killing a human being.
Super Saiyan:
This is a powered-up version of a Saiyan. When a Saiyan reaches this level, their hair turns bright yellow and their eyes turn green. Just about all the important Saiyan reached this level which includes: Goku, Vegeta, Gohan, Trunks, Goten, and Brolli. The requierments for getting this strong are simple: The Saiyan must reach a power level over 100,000 and for him to reach his first transformation, he must have extreme anger. Otherwise, the transformation will not take place. After a Saiyan reaches the Super Saiyan level, it'll be very simple for him to transform afterwards.
Ultra Super Saiyan:
An Ultra Super Saiyan is a buffed-up version of a normal Super Saiyan. When this occurs, a Super Saiyan almost reach level 2. Muscles increases, but speed decreases when the Saiyan is in this form. Brolli and Trunks make this form, but Vegeta can't make it all the way to this stage. Vegeta knew that the Ultra Super Saiyan would slow you down so he didn't go all the way. That way he could still keep his speed. When Brolli went Fully Ultra Super Saiyan, I was impressed. His mass increased incredibly andhe was faster than he was before!!. For a Saiyan to reach this version, The Saiyan has to be in a very high power level. A Saiyan doesn't have to have extreme anger to reach this form.
Super Saiyan 2 :
This level is more powerful than a normal Super Saiyan. When a Saiyan reaches this level, their hair turns golden yellow, they grow more hair and their hair get into triangle-like shapes, their eyes turn bluish green, and their muscles increase more. The only 3 Saiyans were to reach this level which were: Gohan, Goku, and Vegeta. Gohan reaches this level when Cell pushed Gohan over the limit by sending Cell jr.'s to attack the Z fighters and when Cell killed Android 16. Goku reach this level when he was in a battle with Vegeta. Vegeta reaches it when he gets transformed into Majin Vegeta.
Super Saiyan 3:
This stage is more stronger than the 2 other Super Saiyan level. This is an odd looking level for the Saiyans. The Saiyans hair goes down to their feet, eyebrow ridges pushed out and eyebrows completely gone. All of this immense equals a source of power. Goku and the fused Z fighter Gotenks are the only Saiyans to reach this level.
Golden Oozaru:
This is the next level of the Oozaru ape. Golden fur all around them. This happens when a Saiyan see planet earth from planet plant. The Golden Oozaru's power is much more powerful than the original Oozaru. A Saiyan must reach this stage first before he can go to the next level of the Super Saiyan stage.
Super Saiyan 4:
This level of Super Saiyan is only seen in Dragon ball-GT. This is not an actual Super Saiyan, but it's some sort of mix between Oozaru and a normal Saiyan. In this level, the Saiyan keeps their hair color and grow red fur across some of their body (Instead of having his natural hair color and red fur, SSJ4 Gogeta had brown fur across his body and has Red hair). Most say that this level is the true form of the Super Saiyan because you have to reach the Golden Oozaru stage to reach this level of Super Saiyans.