This Is The Original Stage of a Saiyan.They have Long, Black, Spiky hair with a tail. This stage uses up the least energy and usually no energy at all. The only true saiyans still alive are Goku and Vegetta, although Nappa and Raditz where also Saiyans when they where alive. When you are a Saiyan you can transform into an 'Oozaru' if you still have your tail in good condition. Some half Saiyans include Gohan, Trunks, Goten and there are a few others. Saiyans power increase every time they fight and most Saiyans, if not all, have a thing about fighting. They cant go very long without a fight to keep them from boredom!

Super Saiyan is the stage reached after Saiyan. It's reached by having to cope with too much stress, Thus letting alot of anger and rage escape. That Triggers the awesome transformation. The transformation lets your body handle alot more power and speed. Your muscels also get much bigger. Physicly, Your hair turns a golden, blond and your eyes turn green. Gohan, Goku, Trunks, Vegeta, Brolli and Goten can all turn Super Saiyan, But Goku was the first to do it.
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Not to be confused with Super Saiyan 2, this is the level "which surpasses the level of Super Saiyan." When Goku trained in the room of time during the Cell saga, he discovered that he can increase his power almost 3 times more than normal. But he realized that while this increase makes him more powerful, it also makes him slower. This is the major weakness which means that it is no good in battle. Trunks realised this when fighting cell, as he culd not lay a finger on him. Trunks was the first to reach Asscended Saiyan but some argue Vegeta was.

The level which is superior than that of Super Saiyan in terms of strength and speed. Gohan was the first to reach this level during the battle with Cell. It is similar in definition to the Super Saiyan in which rage and anger triggers the transformation. The difference physically is a little increase in hair (about 2 inches) and small electric waves surrounding the body. With this increase in powers, strength and speed all increase in sync. Goku, Gohan, Vegeta And Brolli can all reach ssj2.
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Quite different than the other levels, Super Saiyan 3 can only be achieved through training. During the time when Goku trained in The Other World, Goku learned how to transform to the third level of Super Saiyan. It brings even more power and speed. Physically more hair and the loss of eyebrows is seen in this transformation. The biggest weakness at this level is that it uses more energy, so a Saiyan can only transform to this level for a short time. This is the strongest level of Super Saiyan to be seen in DBZ and is my personal favourite. Goku was first to be able to transform in to a Super Saiyan 3 and was the only person to do it with out having to fuse. Gotenks could also reach SSJ3 when fused.
See An SSJ3 Clip

In Dragonball GT, this is the true level of the Legendary Super Saiyan. Rage, anger, and sorrow all triggers this transformation where physically the hair is black and the body is covered with ape like hair. Vegeta And Goku Both reach this Level, But Vegeta needs help from Bulma to do it. He uses the Brute Ray which causes the transformation. when you are SSJ4 you have a tail again and it doesnt use up as much energy aswell. This is the strongest level of Super Saiyan achievable and achieving it is very hard to do!

This is when a Saiyan looks at a full moon. they turn into a huge ape-like creature. This isn't seen after the early episodes of Dragon Ball Z. In Dragon Ball GT, When you are Super Saiyan 4 and you look at the full moon you turn into a Golden Oozaru. I personally believe this is the "True" legendary Super saiyan As this was the form of Saiyan shown when Vegeta first ever mentioned the legendary Super Saiyan in the early episodes of Dragon Ball Z. This is also the coolest level of saiyan, but only if its the Golden Oozaru. Its almost as stron as Super Saiyan 4 and is almost unstoppable! Goku was the only person ever to be a Golden Oozaru.
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This form is when two Saiyans fuse to create one powerful warrior. This can be done by either the Fusion Dance or by wearing the Potarra earrings. The fused warriors created were: Fat Gogeta, Gogeta, Fat Gotenks, Old Gotenks, Gotenks, and Vegetto. When a fused warrior talks, both of the fused peoples voices talk at the same time and is really cool to listen to. The first fusion ever was Goten and Trunks, who created Gotenks (eventually) but Gotenks was very hard to control and was very cocky.
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So What's The "Legendary Super Saiyan"?

Throughout the story line of Dragonball you will find inconsistencies concerning this topic. The legends says that only one person can become the Legendary Super Saiyan. When Goku first reached Super Saiyan he is the only one who supposedly can do this. Later in the series however, we see Vegeta and Trunks reaching this level. To correct this error, the writers inject Brolli (from Movie #8) as the Saiyan who is the "true" Legendary Super Saiyan. Then later Goku achieves Super Saiyan 3 which surpasses that level. To correct this error again, Dragonball GT introduces another level which is Super Saiyan4. Goku is the first to do this. Then Vegita achieved this level through a device created by Bulma. Which level is the Legendary Super Saiyan? The fact is that the writers got caught up in so much definitions, they forget to take in account things that were said in the past. Whatever your argument may be, the truth is that the writers screwed this idea up!