Saiyans can be dropped in as warriors that were sent as infants to cleanse a planet, and therefore have very little knowledge of their past, and possibly their mission, or they can be from the hidden colony and therefore have access to their race’s history.
The last 3 paragraphs detail briefly the story line of the first 3 seasons of the Dragon Ball Z show. For a more detailed description go to the official Dragon Ball Z website.
The Saiyans are an alien race from a planet in the Milky Way Galaxy. For most of their history, they lived in small communities and existed as nomads, traveling across the continents of their home planet. The largest known community of Saiyans during this time is thought to be only around 100 individuals, mostly part of an extended family. Sharing the planet with the Saiyans was a highly advanced and peaceful race, named the Tsufuru-jin. This race possessed advanced weapons and space travel and while they knew about the Saiyans, they believed their technology would keep them safe.
The two races coexisted on the planet for a long time, until the Saiyans began to unite into larger groups. At first only two or three tribes united for better protection from other hostile tribes, but as more and more tribes came together, the Saiyans became an increasing threat to the safety of the Tsufuru-jin. But still nothing was done about them as the Tsufuru-jin believed the Saiyans natural violence and hostility would limit the size of their communities. Then a leader emerged among the Saiyans, a powerful warrior named Vegita, who united nearly all of the Saiyan tribes into a single army within a few short years and began to wage war against the Tsufuru-jin. In the first few battles, the Saiyans quickly decimated the other race’s defenses, but the Saiyans' technology was primitive and the Tsufuru-jin found ways to match many of the Saiyans' natural abilities. Then a full moon occurred, which for this planet was a very rare occurrence because of the alignment of the planet, the moon, and the local star. All of the Saiyans suddenly transformed into the powerful Oozaru and destroyed every last remnant of the Tsufuru-jin in a single night of slaughter and destruction.
The Saiyan warrior Vegita became the ruler of the Saiyans and the planet was renamed Vegita in his honor. The Saiyans began to adapt the remaining technology that the Tsufuru-jin left behind to their own purposes and they began to explore outside of their planet and into their star system. Time passed uneventfully for the Saiyans as they went out and explored nearby star systems and created small colonies. The Saiyans developed a warrior class-based system, each class represented a different level of strength and skill in battle; lower-class Saiyans were not as skilled or as powerful as the upper-class Saiyans. The lower-class families were considered to be weaker and no special effort was made to train them beyond the basic level of training that all Saiyans received. Although it was possible to advance from a lower-class to an upper-class position, this route was hard to take and often blocked by skeptical upper-class Saiyans. In addition, since the formation of the warrior class-based system, no upper-class family was ever moved to the lower classes even though a few so-called "upper class" warriors were known by the lower-classes to be inept or weak.
This was when the powerful being known as Frieza discovered the Saiyans and enlisted them into his ranks of soldiers. Frieza used the Saiyans as galactic pirates to cleanse planets of any intelligent life and prepare them for sale on the galactic market to any race that wished to colonize it. The Saiyans became feared throughout the Milky Way Galaxy as Planet Pirates. Even a few Saiyans were capable of destroying an entire planet’s population with very little effort. The Saiyans were happy to serve Frieza; he gave them the chance to fight and gave them greater opportunities to become stronger and to test their abilities against other races.
Then Frieza learned the story of the Super Saiyan. A 1000 years earlier on a far away planet, a powerful Saiyan warrior transformed into a Super Saiyan and transformed again during a full moon into an Oozaru and went completely out of control and destroyed the planet that it was on, killing itself, several other Saiyans, and the entire planet’s population. The sheer power of the creature frightened Frieza, and the possibility that another Super Saiyan could emerge from the race made Frieza determined that the Saiyans must be kept under control at all costs. To begin this process, Frieza forced the current ruler of the Saiyans, King Vegita, to turn over his only son, Prince Vegita. King Vegita resented the power that Frieza had over his people and became determined to overthrow Frieza. King Vegita personally led the assault on Frieza’s spaceship the night that Prince Vegita was taken from his home.
It is also important to mention that all the rulers of the planet Vegita were named Vegita to honor the original leader of their species. Only the original heir to the throne is named Vegita, but if the original heir dies and a new heir is chosen, that Saiyan’s name is changed to Vegita. There has never been a female ruler of the planet Vegita.
During the Saiyans' short uprising, all of Frieza’s troops on the planet Vegita were destroyed and King Vegita and a group of his most powerful warriors battled their way onto Frieza’s orbiting spaceship. Only King Vegita made it to Frieza’s throne room, determined to destroy the monster that had virtually enslaved his race, but the more powerful Frieza quickly killed him. Frieza then left his ship to face the army of Saiyan Warriors assembled in orbit of the planet. A lower-class Saiyan Warrior of unprecedented skill, named Bardock, led these forces. But even Bardock’s most powerful attack could not harm Frieza. Frieza used the failed uprising as an excuse to destroy the planet Vegita and to eliminate the entire race of Saiyans.
All of the Saiyans were believed killed except for Prince Vegita, Nappa, Raditz, and Kakarott. Prince Vegita was onboard Frieza’s ship and did not know what had happened to his planet. Nappa, the commander of the now destroyed Saiyan army, along with a lower-class warrior, and the first son of Bardock, known as Raditz, had been cleansing a planet for sale. Kakarott, the second son of Bardock, had been sent to Earth as a baby in order to cleanse it, but had been injured and had lost his memories and programming. He became known as Goku and became the champion of Earth and eventually became one of the most powerful beings in the universe. However, other Saiyans did survive the destruction of the planet of Vegita: there were some sent as babies to cleanse planets, as Kakarott was, but their records had been lost and there were a few hidden colonies that managed to survive without support from their home world.
Eventually the Saiyans did have their revenge against Frieza, although it cost them much. When Vegita and Nappa realized that Kakarott (they refused to call him Goku) was still alive but had not cleansed the planet Earth of the human race, they sent his older brother Raditz to see what had happened. After arriving on Earth, Raditz discovered the Kakarott had become a force for good and had had a half-Saiyan son named Gohan. Kakarott refused to join Raditz in his evil schemes and was forced to kill him with the help of his son Gohan and his enemy Piccalo. After Raditz was killed, Nappa and Vegita came to Earth in order to force Kakarott to join them, but the defenders of Earth fought back alongside Goku. Nappa was killed and Vegita was forced to leave the planet, badly injured.
It was on the planet Namek where Kakarott next met Vegita and first met Frieza. Vegita, Frieza, and Gohan had all traveled separately to Namek in order to use the Dragon Balls located there. Gohan, along with the martial artist Krillin and their friend Bulma, wanted to use the balls to resurrect their friends that Nappa had killed on Earth, but Frieza and Vegita each wanted to use the balls to become immortal so they could rule the universe forever. Eventually, Vegita joined forces with Gohan to fight Frieza, his henchmen, and the powerful mercenary group known as the Ginyu Force. When Kakarott arrived on Namek he saved the life of his son, Krillin, and Vegita and destroyed the remaining members of the Ginyu Force, but was badly injured in the process. While Kakarott was healing and Vegita was resting, Gohan and Krillin used the Dragon Balls to resurrect Piccalo and bring him to Namek in order to fight Frieza.
The battle against Frieza was long and he proved to be too powerful for Piccalo, Gohan, Krillin, and Vegita combined, but Kakarott had been given neough time to fully heal. During the battle against Frieza, Kakarott achieved what so few Saiyans had ever managed to do: he became a Super Saiyan. This terrified Frieza who believed the strength of a Super Saiyan was close to his own. Frieza still believed he could win the battle and continued to fight Kakarott, but when defeat appeared inevitable, Frieza destroyed the planet Namek in hopes of surviving in space while Kakarott would suffocate. But Kakarott killed Frieza before the planet was destroyed, and while he was nearly trapped on Namek when it was destroyed, he survived and returned to Earth. The galaxy had been freed from Frieza, and Vegita went throughout all of the bases that he knew Frieza controlled and destroyed all of his remaining loyal troops.
Saiyans appear to be almost completely human. They have dark hair, such as black, brown, and dark shades of red. Their hair grows naturally very long, but it can be cut often to place it in any style that human hair can manage. Their eyes are either brown or black, but otherwise look like a normal Human's eyes. Their main difference from humans is their monkey-like tail. The tail is prehensile and can be used to pick up objects, attack enemies, push buttons, and similar things. The tail is the same color as the Saiyan’s hair.
Saiyans vary in size from as small as 4’ to as large as 8’ (1.2 to 2.4 m) and they have an incredible appetite and can eat a huge amount of food in a single sitting. A single Saiyan could eat several Thanksgiving dinners one after another and not be full when they were done. Saiyans have to eat ten times the amount of food Humans need, but they only have to eat a fifth as often. Where the average human could go a day without eating, a Saiyan could go five days, but would have to eat more in order to sustain their higher metabolism. Another feature in full blooded Saiyans is that there hair does not grow once they reach adulthood. If for any reason their hair is lost or removed, it will never grow back. This applies to all of their body hair, not just the hair on their heads.
Saiyan children develop rapidly, both physically and mentally. But after reaching age 4, they develop rapidly and quickly surpass even an full Human adult. Although they increase in height and develop sexually at the same rate as a human, they develop faster in everything else. This is what led to the habit of sending Saiyan babies to weakly defended planets. The babies would develop and grow for a few years on the planet and when they reached the age of 4 or 5 they would begin to systematically exterminate the planet’s intelligent population. Saiyan babies were programmed to be very aggressive and the space pod they were sent in would continue to teach them how to fight and would help them progress to their goal of exterminating the planet’s population. After the mission was completed, the child would return to the planet Vegita in the space pod they were originally sent in.