One of the details that started to provoke a great fuss in Dragon Ball was the tail of the little Son Goku. At first it seemed just only a special detail that Toriyama used to give Goku a distinctive mark, but soon we discovered that this tail concealed a secret!
When Goku and company are taken prisoner by Pilaf, our hero tells his friends that his grandpa was killed by a creature appearing during full-moon nights. Bulma and Yamcha think that's an old legend, but when Goku says that his granpa had forbidden him to look at full moon, they both start to suspect about him and... his tail.
Then Goku looks at the full moon and soon becomes unconscious and turns a giant ape, without his reason and control on himself and with an unbelievable destructive power. Goku stops to look like a giant ape and returns normal when they cut his tail off and, when he wakes up, he doesn't remember anything.
This same transformation happens another time during the final match of 20th Tenkaichi Budokai, when Muten Roshi, disguised himself as Jackie Chun, decides to destroy the Moon to avoid this situation to happen again in the future.
A few years after Gohan's birth, here is Raditz coming on Earth and they discover that the tail is a Saiya-jin distinctive mark and the source of Saiyans power. So Goku is a Saiya-jin too, just like his brother! When Goku and Raditz die, Piccolo, aware about the next arrival of other Saiyans, decides to personally train Gohan and it's revealed that also the child "suffers" from the same problem of his father: when he looks at full moon, Gohan turns a giant ape having no self-control or awareness.
Finally all the truth about this transformation is revealed when Vegeta and Nappa come on Earth and fight against Goku. Vegeta, realizing Goku's unexpected strengthening, decides to use his last resource: the transformation into Ohzaru.
But Piccolo had already destroyed the Moon to stop Gohan's fury, so also Vegeta should not be able to turn... Well, he doesn't waste any time, no matter where they've put the moon for this planet, he can make due without it... Vegeta tells Goku that the absence of the Moon will not prevent him from turning and then proceeds to explain that : "The Moonlight indeed is the reflex of ray of Sunlight (in fact we know that the Moon shines with reflected light!)... but only when Sunlight is reflexed in the Moon, the Brute (Burutsu) wave forms... And if the rays of Sunlight are reflected in the full Moon, the power of the Brute wave reaches 17 million zenos... When eyes absorb this wave it's possible to transform... Every planet has his own moons, but only if they're full moons, no matter their size, they are able to emit a Brute wave more powerful than 17 million zenos".
Then he starts creating a powerball within his hand, and more tells "... some Saiyans are able to make a little artificial moon more powerful than 17 million zenos! This is possible by mixing the oxygen existent on the planet with this powerball!" No sooner said than done, he throws the sphere of energy in his hand and, when it reaches a suitable height, he makes it to explode.
Then Vegeta looks at the artificial moon he made and his transformation begins.
And here are some important details about this transformation.
Saiya-jin children aren't yet able to have self-control (when they turn Ohzaru), so they act following their own destructive instinct and, when they return normal, don't remember anything. For this reason, they're subjected to a special training in order to control their own transformation and, in this way, they manage to have self-control and full awareness and are also able to talk. The Saiyans, by turning Ohzaru, become 10 times as powerful as before.