Gohan- Gohan is the first son of Goku.  He is half human and half Sayin.  He is also one of the strongest, although he is very young when most of the battles take place.  

Vegeta- Vegeta is the only other Sayin to have survived the planet's destruction.  He is a prince among Sayins, and in being so, he is very arrogant.  Originally a bad guy, he transfers to the side of good, but always remains Goku's rival.  He eventually gets married and has two kids.

Piccolo- Piccolo hails from the planet Namek.  Even though he is green, he only finds this out half way though the series.  He was also a bad guy in the beginning, but he becomes friends with Goku and ends up training his son, Gohan. 

Trunks- Trunks is Vegeta and Bulma's son.  He comes from the future and warns everyone of Cell, the evil android.  When he grows up in present time, he is a bit more spoiled, but is a lot stronger.  He has the ability to merge with Goku's youngest son, Goten to form Gotenks.

Bulma- Bulma is a blue haired girl with a personality that won't allow her to shut-up.  Even though she isn't a fighter, she is important to the team because she makes gadgets such as the Dragon radar.  Despite their apparent differences, Bulma and Vegeta had two kids, Trunks and Bra.