Saiyans: The Saiyans, who lives from the now-destroyed Planet Vegeta, are possibly the most powerful race in existence. There life-style is one of violence and constant battle, where peaceful pursuits are scorned. There power is such that Freeza ended up destroying the Saiyan home planet in an attempt to eliminate the Saiyans as a possible threat. Saiyans are a powerful race of humanoids. The gravity of their Planet Vegeta is 10 times that of Earth's. They are also physicallyvery tough, and can survive a great deal of punishment. Saiyans (in general) have black hair and brown monkey-like tails.

Oozaru: During a full moon, a Saiyan can transform into a huge 60' tall were-monkey, a form capable of demolishing a city with ease. Several of these were-monkeys can easily wipe out the population of a continent or even a whole world. Cutting off the tail will cause the Saiyajin to return to his (or her) normal form. Tearing this tail out (by the roots) will eliminate the chance of a Saiyan becoming a were-monkey as well as allowing the Saiyan to assume Super-Saiyan form.

Super-Saiyan-1: If a Saiyan has their tail removed (torn out by the roots or just cut), then that Saiyan can assume a high-energy state called 'Super-Saiyan'. This form of Super-Saiyan is marked by the Saiyan in question assuming an aura of energy about the body. The hair goes yellow-gold, the eyes go bright blue and the body bulks out a bit. The requirements for becoming Super-Saiyan (SSJ) is that the Saiyan in question gets angry, very angry and release this anger in form of rage. Goku is able to reach this state after seeing Freeza vaporize Krillin, while Future Trunks assumes SSJ by concentrating on his anger at the death of Gohan. Vegeta goes SSJ by training in the 400 G room. A Saiyan can maintain the SSJ-1 form for hours if needed.

Super-Saiyan-2: This form was discovered and used by Vegeta and Trunks after they finished their stint of training for a year (actually one day) in the Room of Spirit and Time. A Saiyan must have reached their maximum SSJ-1 energy level before assuming SSJ-2. Upon reaching SSJ-2, the Saiyan's hair gets even longer and more spiky, the aura is accompanied by electrical discharges, and the body gains an immense amount of muscle mass. In the Dragonballz series Gohan is the only one that masters this stage to perfection even better than his father. The main problem with SSJ-2 is that the body ends up with so much bulk that one's agility is limited. Vegeta was unable to touch Perfect Cell, much less harm him. Trunks suffered the same problem. Goku assumed SSJ-2 in the Room of Spirit and Time, but recognized the limitations of SSJ-2, and discarded it, by concentrating more on increasing his agility in the SSJ-1 stage. This helps his move quicker in the SSJ-2 stage.

Super-Saiyan-3: According to Goku, this from was only reachable by him after he had died. But, Gotenks is also able to assume the Stage 3 form. Anyway, it is accompanied by massive electrical discharges and bolts of lightning playing across the body. The face becomes thin and untamed, while the hair falls down past the waist in a thick mass of golden-yellow strands. This form is very powerful (but only useable for a very short time).
Super-Saiyan-4: This form is assumed by Goku and Vegeta in Dragonball GT. I am going to not bother about trying to define it. But here is what little I know about this level. In this stage Goku is capable of shattering planets with ease and can probably wipe out a whole solar system single-handedly. The Saiyan regains his tail in this form.