This information is from: Super Saiyan UK
The Super Saiyan form is first reached by Goku in the Frieza saga,
after Krillin had been killed. This made Goku extremely mad, and thus
changed into a Super Saiyan. This form increases your power level
immensely. Muscles increase in size, hair turns golden and eyes turn
into an aqua colour. To reach Super Saiyan one must be of Saiyan blood,
Gohan and Trunks are only of half Saiyan blood, but both become
Super Saiyans. Also one must be of pure heart, Vegeta pointed out that,
a Saiyans heart can be pure good or pure evil. Finally one must reach an
extreme rage. Goku was raged because of the death of Krillin, Trunks was
raged because of the death of Gohan in his world, Vegeta was raged becuse
he was surpassed by Goku and Trunks in power, and Gohan was raged by
recalling the times when he could have saved the day but wimped out and
caused his friends deaths.
There are 4 grades of the first Super Saiyan form:
The first grade is the one mention above when Goku first reached
Super Saiyan. The second grade is first shown by Vegeta after training in
the Hyperbolic Time Chamber, it utilizes more of the Super Saiyan power.
The third grade, or Ultra Super Saiyan, is the most powerfull of
the muscles hugely increase in size, and this is the down point.
The muscles are so big that speed is lost, and as Goku told Gohan,
Power is nothing if you can't hit your opponent.
The forth and final grade
is Shown by Goku fighting Cell. Super Saiyan Full Power can maintain
their form and act normally, it fully Utilizes the Super Saiyan power
and is a perfect balance between power and speed.