Dragonball Starts out about bulmas Quest to find the dragonballs.Along the way she finds many alies and eventually the show in dragonball turns into goku being the main character of the show.Dragonball z is about mostly about gohan growing up and about having to fight frieza,cell,and buu.It is also about turning Super saiyan-a form and as always saving the earth.
Dragonball GT is about goku being wished back to a kid by pilaf{a old enemy} and having to find the dragonballs again.
That is the simple way of describing DB/Z/GT.