He's the main character for most of the series (Gohan "takes over" the series when his father dies the second time). He starts the manga as a young and innocent boy with black spiky hair, a strange monkey tail and an expanding stick (nyoibo).
Incredibly strong for his age (which he claims to be of 14 years but later we find out that he can´t count), he has one of the Dragonballs which he refers to as his Grandfather (it was the only thing that his Grandfather left before dying, so he considers it to posses his soul). Gokuu while carrying his lunch home (a giant fish that he killed with one kick) literally bumps into Bulma. After a brief discussion, and Bulma explaining the power that lies in the Dragonball, both leave to search for the Dragonball. It is in this way that the series of adventures begin. Soon after, both meet Kamessenin who then gives him his "first" magic cloud ( he frequently has it destroyed, but later we find out there's a bunch of them), also known as Kintoen.
After some 17 mangas, Gokuu has had many adventures and friends and is already an adult, married (with ChiChi) and has a child (Songohan, he was given the same name as Gokuu's Grandfather). - On the TV series the name is changed to Dragonball Z- . By this time, with the coming of Gokuu's brother: Raditz, we find out that Gokuu isn't human. He was found by his "Grandfather"(SonGohan isn´t his real grandfather, but an adoptive one) inside a space capsule; Gokuu had been sent to Earth in order to conquer it, but due to a big fall he lost his memory thus, becoming the innocent boy that we meet at the beginning of the manga; we are told that Gokuu is really a Saiya-jín (-jin is used in Japanese to say that a person is part of the people that the "-jin" follows, therefore, Gokuu is part of the Saiya people; living the Saiya in planet Vejiita (they are not originally from this planet-this we find out in one of the films).
Later Gokuu finds out that he can turn into Super Saiya-jin when he gets really angry (this occurs the first time when Freeza, one of the most dangerous enemies during the series, kills Kulilin, during the quest for the Namekian Balls). This transformation increases greatly Gokuu's power and gives him physical transformations (his hair becomes blond and more spiky, his eyes green, his face linear and an aura forms around him); afterwards, he finds that he can also turn into Super saiya-jin 2 and 3, which means an increase of all the caracteristics of the first stage.
Note: Songokuu is inspired in the monkey god of the Shinto religion (SonGokuu really looked like a monkey in the first Toriyama sketchs for Dragonball) .
Son of Songokuu and ChiChi, he appears the first time at the age of 4, about the same time that Raditz appears (beginning of the anime series Dragonball Z). Having been given the same name as SonGokuu's "Step-Grandfather", SonGohan can become more powerful than his father (because he is half-saiya-jín -->hybrids are stronger). However, ChiChi is always telling him to study, therefore, he never really explores his fighting capabilities or gains the taste for a good fight. Gohan has his first fight training with Piccolo, after Gokuu's death. During the fight against Raditz, Piccolo decides to train Gohan and sort of allie with Gokuu against Vejiita and Nappa. Though Piccolo is very harsh with him during the training, Songohan and Piccolo become friends, being Piccolo an idol to Gohan (Piccolo finnaly got killed by Nappa in an atempt to protect Gohan). This motivates the participation of Gohan in the Quest for the Namekian Dragonballs in a chance to revive Piccolo. Songohan is often seen during part of the series using clothes much like those worn by Piccolo.
Gohan becomes the main character after the Cell Saga (during this time Gokuu is "dead"), assuming a diferent identity with which he fights against Crime (he was first known as The Golden-Haired Warrior - he gets his "instant" costume from Bulma (when pressing a button on his watch an instant costume appears), using his transformation into Super Saiyan and sunglasses as a facial disguise, but because the disguise isn't very good he finally decides to turn into: The Great Saiya-man, using this time a helmet; he spends most of the time praticing poses instead of fighting crime...
At the end of the series Gohan becomes a scholar in a local college (he looks a little nerdy). He joins (I don't really know if they marry) with Videl (also known as Beederu). This union results in a daughter, Pan.
Saiya-jin with hair about his waist length. He is the first enemy to appear in the Dragonball Z series. Songokuu's older brother comes to Earth in the belief that his brother has conquered the planet. After knowing that Gokuu has lost his memory, he kidnaps Gohan demanding that Gokuu presents, the next day, 100 dead humans on the beach in front of the Kame House, or else, he will kill his son (Gohan is 4 at the time). Gokuu and Piccolo (Piccolo wanted to be the only one to get to kill Gokuu) join forces and fight Raditz. Raditz is much stronger than any of the them, so Gokuu has to grab him, while Piccolo performs his "Screw" Attack, this attack goes through both Raditz and Gokuu body, which causes the death of both.
Vejiita
Prince of the Saiya-jin, appears after the death of Raditz. He comes with Nappa to Earth in an attempt to gain immortality by wishing it to Shen-long. He appears first with brown hair (in the anime) and slightly different looks, but after a year, when he comes to earth, his looks are already the same as those that we see for the rest of the series(Vejiita seems to be about 10 years old in his 1 year space journey). Vejiita finally joins Gokuu and friends but never gets to be a real allie; he's only purpose in life is to defeat Gokuu and prove that he is stronger. During the series he slightly settles down with Bulma having with her two kids: Trunks and Bura
Tall and bald Saiya-jin comes to Earth with Vejiita seeking immortality (he became aware of the existence of the Dragonballs when Raditz came to earth, in search of his brother -Kakarot- knowned as SonGokuu). While on Earth he kills Chaoz (he actually "self-explodes" in an attempt to kill Nappa). In addition, he also kills Tenshinhan and Piccolo (Nappa was going to kill Songohan but Piccolo stood in the way of the blow) before Gokuu could help. Gokuu finally appears and he is defeated. Vejiita didn't like the weakness of his partner so he kills Nappa.
Nappa had with him the Saibaimen (see the second image), seeds that when planted give "birth" to 6 humanoids,one of these killed Yamcha, but were then all killed by Krilin, except one that when atacking Gohan was stoped and then killed by Piccolo (the saibamen have the special capability to split their head in two and throw acid out of it..
note: Nappa was, in past times Vejiita's caretaker.
Gokuu's and Raditz's father (we know nothing about their mother). Many years aho about the same time that SonGokuu was send to Earth, his father and other Saiya-jin fought against Freeza (Freeza was afraid of the power of the Saiyans ), like most of the Saiyans he was killed by Freeza (this fight motivated the destruction of the Saiyan-jin planet: Vejiita)- so after this, the only remaining pure Saiyans are: SonGokuu, Raditz (now dead), Nappa (also dead) and Vejiita.
Vejiita's and Bulma's son. Appears the first time when coming from the future to warn Gokuu and friends about the coming of Cell and the androids in 3 years (and save Gokuu from dying, who because of the use of teleport has aquired a heart disease. He first appears, when Cyborg Freeza and Great King Cold appear, killing both. He often during the series carries a sword and uses a capsule corp. jacket.
I can say that there are two Trunks:
o The "future" Trunks, who because of his tough life is a lot stronger (in his time he lives in constant fighting so this gives him a lot more training, making him one of the strongest of the series) and also makes him more polite;
In his future all of the earth population (including the Z warriors) had been killed by the androids (except for Gokuu who had died of heart disease provoked by his teleporting tecnique). Hence, Trunks travels 20 years back in time to give medicine to Gokuu and warns the Z warriors about the coming of the androids (he comes back from the future at the same time that Cyborg Freeza and great King Cold appear, killing them both; Trunks knew that by this time, Gokuu would have returned to Earth. Afterwards, Trunks goes back to his own time, but returns again three years later to help the Z fighters battle the Artificial humans. During his stay in the present, he becomes extremely strong, being stronger than any of the Z warriors (excluding Gohan and maybe Gokuu). After Cell is killed by Gohan, Trunks returns to the future and kills the cyborgs #17, #18 and Cell (in his future #17 and #18 seemed to be stronger), that terrorized his future.
o The "present" Trunks is not so strong or polite as his future self,- this always gets himself (and his friend Goten) in trouble. During the Z warriors fight against Buu, Trunks fuses with Goten becoming Gotenks, a very powerful character.