Gohan

Gohan is Goku's first son. He is introduced to the series when he was four years old. At the end of the series he is roughly 26.
Gohan is the most powerful character in the Dragonball world but lacks the love of fightning that most Sayians have. Gohan is the first character in Dragonball to reach Super Sayian 2 during the Cell series. His power is awakened when he sees his friends being beaten and killed. The death on Artificial Human #16 was what finally made him snap. It was also about this time that the main characters switched from Goku to Gohan.

Vegita

Vegita is, personally, my favorite. He is the bad guy among the good guys. He first appeared back when Gohan was four. Although he hangs with the Z fighters he still tries to be bad. He is the prince of all Sayians and has quite a large ego. He is always trying to prove that he is the best fighter in the universe.
Despite the fact he is always trying to be bad, Vegita has changed over the last while. Deep down he is caring about his friends and expecially his son Trunks. Although he tries to hid it he also even like Goku a bit even though he is always trying to prove that he is stronger than Goku (like that will ever happen), and even respects him for his power.

Piccolo

Piccolo starts off as Goku's enemy but that changes throughout Dragonball. Piccolo was born just as his father was killed by Goku. In Old Piccolo's last breath he spat out the egg that hatched Piccolo.
Piccolo become uneasy allies as they team up to fight Raditz, Goku's brother. When the fight was over Piccolo took Gohan and started his training. Goku and Piccolo team up again against Freeza on Piccolo's home planet Namek.
As a Namek Piccolo can regrow almost every body part and can stretch his arms to amazing lengths. Goku and Piccolo soon became friends.

Goku (Kakarott)

Goku is the main character in Dragonball. The series started when he was 12 years old. Once he grew up and he had his sone Gohan, the show changed to Dragonball Z. At the end of this series Goku was about 45.
Goku is a Sayian from the planet Vegita. He was sent to earth to destroy the humans and take over the planet. However, when Goku was a child he fell into a large ravine and hit his head. Because of this all the programming that was given to him was erased, making Goku the kind person we see in the series. Goku was raised as a human on earth and did not learn he was a Sayian until his big brother Raditz came to earth.
Goku is the second strongest person in the Dragonball world. Currently he can just go to Super Sayian level 1, but later on he reaches Super Sayian level 4.

Trunks

Trunks is Vegita's son. In the series we see two Trunks, future Trunks and the present one. Future Trunks comes from a world where the Z Fighters don't exist. They were all killed by the Artificial Humans with the exception of Goku who died from a Heart disease. Trunks travels back in time 20 years to deliver medicine to Goku so he wouldn't die. When he arrives is the first time he ever sees his father Vegita. When he had delivered the medicine he went back to his own time but returned three years later to help fight the Artificial Humans.
The main diference between future Trunks and present Trunks is the attitude. Future Trunks is very polite while present Trunks has an attitude more like his fathers.

Kryllin

Kryllin has been Goku's best friend since they were very young. They both trained under Master Roshi and have been together through everything.
Kryllin, although not one of the strongest fighters will never give up during the fight no matter how bad it looks. He stuck around during the fight against Vegita, although most his friends had been killed. He stood up to Freeza even though he knew he could not beat him and he stands with his friends in almost every fight they have been through.
It was Kryllin's death that pushed Goku over the edge in the Freeza battle tramsforming him into a Super Sayian.