Goku
Goku is the primary hero of the Dragonball series. The series started out with Goku as a child of age 12. Once he grew up and his son was born, the TV series' name changed from Dragonball to Dragonball Z. At the end of the series, Goku was roughly 45 years old. Goku is a Saiyan (or it is spelt Saiyajin but I prefer Saiyan for short) whose native world is the planet Vegeta. He was sent to Earth in a space capsule right after he was born so he could conquer the planet. However, he hit his head and lost his "programming," thus becoming the kinder, gentler Goku we see in the series. Goku was raised as a human on Earth by Gohan (not his son), and did not learn of his true nature until his brother Raditz came to Earth. He detests being a Saiyan and considers himself a human. Later, when Goku's rage was awakened by witnessing the death of his friend Krillin his Super Saiyan power was awakened. When he goes Super Saiyan his eyes turn green and his hair gold. Currently, Goku can go to the 4th stage of Super Saiyan power.
Gohan
Gohan is Goku's first son. At the end of the series he was 26 years old. For a short while after the second time Goku died in the series, Gohan became the primary character. Gohan is the most powerful non-fused character in the DBZ series, but lacks the love of fighting his father has and thus rarely uses his full strength. Gohan is a very good-natured boy and unlike his father, hates fighting, although for a while he was more powerful. He is truly one of the most poweful beings ever. He is very polite towards just about everyone except to his enemies. He has an extreme power sleeping inside of him that is awakened whenever something he loves is threatened. He is a force to be reckoned with whenever enraged. Everyone always underestimates him as a kid but he uses his sensitivity to his advantage. Gohan is very bright and studies very hard, although this is somewhat due to the wishes of his very strict mother, Chi-Chi. Gohan is the first character in the series to go Super Saiyan level 2, during the Cell Game; the power of his anger was kindled by watching his friends get beaten up, and the death of Artificial Human #16 sent him over the edge. It was roughly at this time that the series switched main characters from Goku to Gohan.
Piccolo
Piccolo is Goku's old rival. He was born after Goku killed his father when he was a kid (accidentally by turning into the monstrous ape in the full moon); in his last breath the old Piccolo spat out an egg, and the new Piccolo hatched. Sick huh. That is one of the things rumored to how Piccolo was made but the true reason how it happened was that Kame was trying to get rid of all the evil in his spirit. Unfortunately he did thusforth creating Piccolo. Piccolo then went on to fight Goku in the final round of the Tenkaichi-Budokai, which he subsequently lost by a very close margin. Piccolo is a native of the planet Namek, although he doesn't learn this for quite some time after his first appearance. As a Namek, he can regenerate almost any part of his body lost. (usually the arms) He can also extend the range of his arms till they're really long. Later, he goes to Namek to help Krillin, Goku, Gohan, and Vegeta fight Freeza after the Tenkaichi-Budokai, Piccolo becomes more and more of a "good guy." Eventually he becomes Goku's friend. Once one of the most powerful characters in the series, he can be considered a "high class fighter," and strong among the Z Fighters. Although he acts like he doesn't care about anyone it is very untrue. Gohan taught him how to have emotions. Piccolo always challenges Goku early in the series but they eventually become friends.
Vegeta
Vegeta is the "bad guy among the good guys." He first made his appearance back when Gohan was 4. Although he fights alongside the other Z Fighters, he is still decidedly evil. As the prince of the Saiyans, he is very proud, has a huge ego, and is constantly trying to prove that he is the best fighter in the universe. Vegeta has a continual rivalry with Goku, or "Kakarott," (Goku's Saiyan name) and is obsessed with beating him in combat just to show that he is the stronger. This obsession with trying to be stronger than Goku proves to be a fairly large weakness as well. Despites his efforts to show otherwise, Vegeta has changed and has developed a conscience, and deep down cares about his friends. Furthermore, he is also somewhat emotional at heart. He demonstrates this when dealing with his son, Trunks, most notably just before he died trying to kill Majin-Buu. Although he pretends to hate Goku with a passion in reality he respects his power and maybe even likes him a bit.
Trunks
Trunks is Vegeta's son. In his future, all of the Z Fighters were killed by the Artificial Humans, except for Goku, who died of heart disease. Trunks travels 20 years back in time to give medicine to Goku so he can survive this disease and defeat the Artificial Humans. Trunks' returning in time alters the past in other ways which can be seen later. After giving the medicine to Goku, Trunks goes back to his own time, but returns again three years later to help the Z Fighters battle the Artificial humans. During this time, he becomes extremely powerful and surpasses every other one of the Z Fighters except for Gohan and (maybe) Goku. The Trunks here, "Future Trunks" and the "current Trunks" are radically different characters. "Future Trunks" is very polite like Gohan and has very strong values, but the current "Chibi Trunks" has a bad attitude and is somewhat like his father. Trunks never truly reaches the power level of his future counterpart; possibly because he lived through mostly peaceful times, whereas Future Trunks did not.
Krillin
Krillin is Goku's best friend, and has been around since the first series. He is a fighting monk who leaves his temple to find and train with Mutenroshi. He is kind of short, has no nose and throughout most of the series, has no hair either. Krillin trained with Goku under Mutenroshi and afterwards shared many adventures with him. Krillin has died more times than anyone else in the series. He is one of the weaker fighters, but this is partially due to his low self-confidence. Nevertheless, he has gotten fairly powerful due to all his fights with the really powerful enemies, and I think is the next strongest Z Fighter after Piccolo. He becomes good friends with Gohan probably because they are almost the same height in the beginning. He is caring and will do almost anything for his friends.
Goten
Goten is Goku's second son, and a dead ringer for Goku when he was a little kid. At the end of the series, Goten was 17 and looked nothing like Goku though. Goten is very powerful for someone his age, although he is a little bit neive like his father. He can fuse with Trunks to become Gotenks, one of the most powerful characters in the series.
Chiaotzu
A master of telepathy, Chiaotzu has the power to stop people in their tracks and to communicate telepathically. This white-skinned, red-checked little being is the devoted companion of Tien. A strong fighter in the early days, Chiaotzu does not hesitate to throw himself onto Nappa's back and self-destruct in an attempt to save his friends. However, his power level does not increase as dramatically as the other Z Fighters. Thus, Chiaotzu is not a major warrior in the later episodes of Dragon Ball Z.
Tien
Tien is a three-eyed man intent on training to be strong. Originally trained by one of Master Roshi's rivals, Tien switched sides and became a Z Fighter. He beat Goku at a worldwide martial arts tournament, is able to stop a waterfall with a blast and can crush rocks on his head. His fighting techniques include the ability to hover in the air, throw concentration bombs, blind with flashes of light, and split into two or four separate entities. Tien is excited about being trained by Kami and willing to fight against the Saiyans despite the odds. He volunteers to be the first to fight against the Saibamen, wins the battle and goes on to fight against the Saiyans. Tien can communicate telepathically with Chiaotzu, his best friend.
Bulma
The second character introduced in Dragonball, Bulma is smart and witty but also goofy. Like her father (the founder of Capsule Corporation) she is very handy with technology. Sometimes she comes off as selfish and irritating and can get on peoples nerves. Eventually she and Vegeta "pair up" and have two children: Trunks and Bra
Yamcha
At first Yamcha was a thief and one of Goku's enemies. But as the story went on he begins to fall in love with Bulma (early in the series they go out together) and thus he starts to develop a friendship with Goku. He is the weakest member of the Z Team.
Chi-Chi
The strongest wife in the universe! Chi-Chi is the wife of Goku. She met Goku when they were young. At that time Goku made a promise to marry her and she does not forget this. When Chi-Chi and Goku meet up again as teenagers Goku doesn't recognize her. Angry that Goku does not remember her, she enters herself in a fighting tournament Goku participated in and fights him before letting him know who she really is. Goku naturally wins the match and by the end of the day they are husband and wife. Chi-Chi and Goku have two children: Gohan and Goten.
Ox King
The fabled "Gyuumaoh", the Ox king is Chi-Chi's father. He appeared early in the Dragonball series. The turtle master knew Ox King as the one who trained with Goku's Grandfather, Gohan.
Kami
Piccolo's spiritual opposite and the Watcher of Earth (literally god). Before he became god, he had to expel the small evil that was within him. When he did this the evil that split from him came to be known as Piccolo's Parent. If Piccolo is in danger and dies Kami dies as well. They cannot exist without each other. Kami is part of an alien race called the Namekians.
King Kai
King Kai is the fun loving comedian who is the watcher of the the north galaxy. He is one of Goku's trainers and taught him the Kaio-Ken, a powerful technique. Later in the series we see that there are other deities that watch different parts of the universe. King Kai is just one of many deities who appear in the series.
Hercule
The comic relief of Dragonball. Hercule first appeared in Dragonball Z as Earth's so called world champion. He tried to fight Cell in the Cell tournament only to be flicked away silly. Many people on earth believed that he defeated Cell and that he is the most powerful. Hercule realizes that he is not the strongest when he bumps into the Z Team and tries so hard to protect up his image during the series.
Videl
Hercule's daughter, and Gohan's one time classmate. During Gohan's first years in high school Videl tried in vain to figure out who the Great Saiyaman is. Eventually she figures out Gohan is the Great Saiyaman and asks him to teach her how to fly. Videl is more powerful than her father. She fights for justice as one of the cities great crime fighters. By the end of the series Gohan and Videl get married and have one child: Pan.
Master Roshi
The old master of martial arts. Master Roshi has strange taste in clothing and a sex crazed mind. He trains Goku and Krillin and teaches them everything he knows. He lives in a house on a small island with his turtle friend.
