Sayian when reach to certain powerlevel or when is at the extreme point of its anger,it transform into the next stage.
Its just like evolution of Pokemon.After transforming,it gets new powers with ultimate speed.
There are five Stage of sayain transformation:
Base form :Every sayian at the first stage of transformation
Super Sayian :Goku was the first to become it.Every mian protagonist became Super sayian.
Ultra Super Sayian :Vegeta was the first to become this.Trunks and Goku later become Ultra Supersayian.
Super Sayian 2 :Gohan was the first to become SSJ2.Goku and Vegeta also reached to this stage.
Super Sayian 3 :Goku was the first to reach it.Gotenks also reached to this stage.
Super Sayian 4 :Goku and Vegeta reached to this stage first.
Super Sayain God :Only Goku is applicable to transform into this stage.

There are other classifications like False Super sayian and Legendary Supersayain but they are limited.
Goku is the only False Super sayian and Broly is the only Legendary Supersayian.

Super Sayain God :Only Goku is applicable to transform into this stage.

I love this Pajama guy

But it is very wierd to insert such things in the anime when it has gone far beyond.

Far beyond?

Yes far beyond.They started Dragon Ball GT after Z.So by Super Sayian God,the whole story changes

The Pajama Boy changes the whole thing.

Yes,that was my most favorite part of Dragon Ball.
Video from : www.dailymotion.com

I watched it right now, lol.Who was that who transformed? Gohan?

Its Gohan.You can't imaging the potential in this boy.

Yup. He is so cool. Can't believe it.

Another Video about Goku becoming Super Sayian.

Video from : www.dailymotion.com
#user_search=1