Regular Saiyan
This is the stage a saiyan is born into. They usually have black hair and have a tail.
Super Saiyan
This stage was first reached by Goku when he was figthing Frieza.
Physical Description: Increased muscle mass and their hair points upward and becomes long & blond. Accompanied by a yellow aura. The power comes in response to a need, not a desire. Goku, Vegeta, Gohan, Goten, Future/Chibi Trunks reached this stage.
Super Saiyan 2
Gohan was the first to reach this while fighting Cell.
Physical Description: Incresed muscle mass and their hair becomes longer and spikeyer. Accompanied by a yellow aura and lighting. Gohan, Goku, Vegeta reached this stage.
Super Saiyan 3
Goku and Gotenks were the first and only to reach this level of power.
Physical Description: Incresed muscle mass and their gets a lot longer. Accompanied by a yellow aura and lighting.
Super Saiyan 4
This stage was first reached by Goku while fighting Bebi Vegeta.
Physical Description: More muscle mass, body is covered with red fur, hair turns black and busheier, and their tail grows back if it was cut off. Goku and Vegeta are the only saiyans to achive this level.
Oozaru
This form is reached when a saiyan looks at any full moon or simulation of a full moon. Any saiyan can undergo this transformation as long as they have their tail.
Golden Oozaru
A level that was achieved by Goku and Bebi-Vegeta in Dragon Ball GT. This is the most powerful form of a saiyan.
Physical Description: Their strength is amazing and their fur is gold instead of brown.