Goku
Goku is the hero of Dragonball Z, a good-natured and trusting soul. He is also the most powerful warrior on Earth and the first Super Saiyan in over 1,000 years. Having honed his skills and strengthened his spirit to a level never achieved by any being on Earth, Goku is able to fly, shoot energy beams from his hands, move at incredible speeds and, in a pinch, power up to a fighting level so high it creates an energy aura around his body. Goku hasn't learned these skills just for fun.
When the Saiyans arrive it is a total shock in more ways than one. Not only does Goku meet his deadliest foe ever, but he learns the mysterious secret of his own past: he is a Saiyan formerly named Kakarot who was sent to destroy Earth. Goku's destiny is altered, however, when he is discovered in the wilds as an infant and raised by a peaceful wise man named Gohan. A near-fatal head injury scrambles Goku's programming. Instead of growing up to become a destructive super-warrior, he becomes innocent and pure of heart, fighting only for good.
Goku is very protective of his family and friends and is always there to help them. If he has a weakness, it is his trusting nature. Goku is always looking for the good in people and can often find it where others do not. This costs him dearly when be believes that the Saiyan named Raditz has turned away from evil. Goku later pays the ultimate sacrifice to rid the Earth of Raditz.
Fortunately for our hero and for Earth, it's not the end of Goku. He is taken to the Upper World to train under King Kai in preparation for the coming of two even stronger Saiyans. The Dragonballs will summon him when the Saiyans approach Earth.
Goku has many powerful fighting techniques including the powerful Kamehameha wave and the Kaio-Ken, which he learns from King Kai. His most powerful weapon is an immensely powerful energy ball. This ball can only be used once and must be guided carefully, for it can destroy the planet if used inappropriately.
Goku's power level changes rapidly, rising from a 300 level in the early episodes to 5,000 after training with King Kai, to 8,000 when he is angry and to 21,000 when doing the famous Triple Kaio-Ken. And all of this is before his Super Saiyan stage. To further aid him, Goku still has the famous Nimbus Cloud and Power Pole.

Vegita
Vegeta is the most powerful and feared of all the Saiyans. He bears the name of the planet from which he hails. Cunning, ruthless and cruel in battle, he enjoys toying with those helpless creatures who dare oppose him. He is a "Super Elitist," the highest class of Saiyans, and believes he and Nappa can conquer Earth in three days. As the mightiest Saiyan warrior, Vegeta has no tolerance for failure. In fact, he has eliminated his own men for showing weakness. After Nappa fails to defeat Goku, for instance, Vegeta mercilessly gets rid of him. Like all Saiyans who still possess their tails, Vegeta metamorphoses into a Giant Monkey when he sees the full moon, increasing his strength tenfold. This is when a Saiyan is at his fullest potential. Arrogant to a fault, Vegeta hates the thought of being weaker than any other creature. Due to this fear he is constantly training and searching for the extra edge that will keep him on top. Vegeta is even more intent on destroying Gohan, for he realizes that the combination of human and Saiyan is stronger than human or Saiyan alone. Physically smaller than Goku, Vegeta has wild black hair that is pushed back by a receding hairline. Nevertheless, he can power up to unbelievable levels and destroy entire planets. When he gathers all the Dragonballs his wish is simple: immortality. While exceeding Goku in both strength and technique, Vegeta does not win the battle for Earth. He makes the mistake of creating a moon to experience the tenfold increase in strength associated with the Giant Monkey metamorphosis. Gohan, however, also sees the moon and subsequently transforms into a Giant Monkey, too. When Yarjirobe sneaks up behind Vegeta and cuts off his tail, stopping the metamorphosis, Gohan takes the upper hand. Vegeta flees Earth. His flight would have been stopped and Vegeta killed except for Goku's intervention.

Gohan
Gohan, Goku and Chi-Chi's four-year-old son, was named after his adoptive grandfather. Physically, he resembles a young Goku, right down to his tail and his propensity to become a raging Giant Monkey when he looks at a full moon. Spiritually, he is good and pure of heart. Because of his protective mother, Gohan begins life as something of a wimp, but when the Earth is threatened, he reveals his tremendous potential as a fighter. In one instance, he uncontrollably "powers up" in order to rescue his father from a desperate situation. Soon Gohan is forced into training in preparation for the eminent arrival of the evil Saiyans. As a result, he becomes a formidable fighter in his own right under the guidance of Piccolo (the former archenemy of Gohan's father, Goku). Gohan first masters survival training. He is abandoned for six months in the wilderness, where he must battle hunger, scorpions, saber tigers and dinosaurs, among other things. Next comes intense fighting against Piccolo to train Gohan in the art of war. His mission: "to kill the Saiyans and save Earth." Gohan learns many lessons under Piccolo, such as "don't see the enemy, feel him." Their special bond results in Piccolo stepping in front of Nappa's mighty blast to save Gohan's life. Initially caught between "wimp" and "warrior," Gohan's loss of nerve in the battle against the Saiyans costs his friends dearly. Haunted by his failure, he is determined to make up for his cowardice. This determination leads Gohan to Planet Namek, where he searches for the source of the original Dragonball so that he can wish his friends back to life. In the end, Gohan comes through in the final battle as he looks at the moon, turns into a Giant Monkey and lunges toward Vegeta, causing him to flee. Gohan's power when initially kidnapped and placed in the spaceship is 710. It jumps to 1,307 when he sees Goku hurt, then to 28,000 when he goes to fight after Piccolo is injured. His tremendous power is credited to his unique genetic makeup, which makes him a stronger being.