In the very early stage of a pure-blooded Saiya-Jin, they all have black hair and they look just like ordinary people, except they have a tail at their rear end (like a monkey). They gain their power level by training, fighting and improving. The maximum power level can be gained by an exceptionly powerful Saiya-Jin is around 20,000 or 30,000. However, another way is to gain power by looking at a full moon (Ozuro Mode). By looking at a full moon will transform them into a giant monsterous ape, which the power level is increased 10 times.

Advantage:
When a Sayian-Jin is turned into an ape (Ozuro Mode), they gain great stength.
Disadvantage:
If the moon is suddenly covered up by dark cloud or obstructed by a certain object, the Saiya-Jin ape is instantly instantly transformed back to a normal Saiya-Jin.
Interesting Fact:
Trunks/Future Trunks and Ancient Saiyan Brolly are the only Saiyans don't have black hair. Trunks hasn't got black hair because he is mixed (son of Bulma and Vegeta).
In the very first battle between Gokou and Prince Vegeta, Gokou got his ass kicked by Vegeta when Vegeta is turned into an ape.
Gokou's tail was removed when he was with Bulma and Yamcha, Yamcha had to cut it off since Gokou transformed into a giant ape.
Gokou was born to be a much inferior Saiya-Jin, with a power level of only 2.
A Saiya-Jin cannot turn into an ape if he/she hasn't got a tail. The tail is where the energy is collected to transform into a huge ape.