Goku is the main character of the show.
Originally sent to earth as a kid to destroy it, but he turned to the side of good when he had an accident and hit his head. Goku is one of the 4 Saiya-jins to survive the explosion of the planet Vegeta. Goku is trained by Kamesennin (Master Roshi) at the age of eight and is sent on a mission in the very first Dragonball Saga to recover the Dragonballs. He defeats the entire Red Ribbon Army by himself, with the exception of Dr. Gero. He later on settles down and marries Chi-Chi, and they have a son named Gohan.Son Gohan
Gohan is the son of Goku. He plays a vital part in the Furiza Saga.
He never fears his enemy even if outpowered.
Gohan was trained by Piccolo at an early age. After Piccolo got killed on earth (as well as Chaozu, Jamcha and Tien). Gohan, Krillin , and Bulma fly to the planet Namek to wish their friends back (easier said than done). Gohan is a strong fighter but his mother Chichi always seems to keep Gohan from having fun by making him study all the time. In DBGT he marries Mr. Satan's daughter Videl and they have a daughter called Pan.
Cell, one of the strongest enemies in DBZ. He makes his first appearance in the Android Saga. His main ability to gain strength in his first form by absorbing people with his tail. After he absorbs 17 and 18, he finaly reaches his perfect form and becomes almost unstoppable! He gets killed by Gohan when he turns Super Saiya-jin 2Trunks
Son of Vegeta and Bulma (two children in all, later he has a sister named Bra).
Trunks plays a vital part during the Android Saga. He first shows up at the age of 14 in the episode: "The Mysterious Youth". Trunks kills off Furiza as well as King Cold. His real reason for his appearance is to warn Goku of the androids ahead of time so they can get prepared. He also gives Goku a medicine that will help him survive the heart virus that he will get three years in the future. After all the talking with Goku is done he heads back to his own time.
But after three years he returns once more to check up on the others, only to find out that the androids in his time are more different then the androids the Z fighters are fighting now.Piccolo
Piccolo is the evil side of Kami-sama. In the old DB episodes Kami-sama wanted to take the space as guardian of earth, but first he had to get rid of all his evil by splitting away his evil side which became Piccolo.
During the first Dragonball Series, Piccolo was Gokus number 1 enemy because he kept getting his butt kicked by Goku.
But in the DBZ Series he teams up with Goku and trains his son Gohan. Piccolo even sacrifices his life in the Saya-jin Saga in order to save Gohan. He later fuses again with Kami-sama in order to be powerful enough to fight the androids.Vegeta
Vegeta is the prince of all Saiyans, literally speaking. His father was King Vegeta, ruler of Planet Vegeta. He is the Goku's Nemesis. Vegeta was Furiza's henchman until he decides to find the Dragonballs for himself. Vegeta's only goal is to become a super saya-jin. But it will take until the Android Saga until he finally reaches his goal. He is a very evil character who has no remorse or feelings for anyone. He gets very egocentric during the Android Saga when he reaches the point of being a Super Saiya-jin. His determination of being stronger then anyone else usually get him to a point of almost being killed by his opponent.
Vegeta is also the father of Trunks.Krillin
Krillin, Goku's best friend and Gohan's companion in the Freeza Saga.
Krillin and Goku knew each other from an early age. He never seems to fear his enemies and always helps in anyway possible. He is sometimes under estimated because of his height, but he truly is a great fighter. For some weird reason Akira Toriyama made a dramatic character change in Krillin during the Android Saga. Krillin takes Gohan's old role as a coward on the battle field who sticks and hides in the background. This is a very anoying change since many of Krillin's fans see him as a brave fighter who never fears a fight.