Dragon Ball is a comedy/action manga starring a 12 year old boy named Goku.
Dragon Ball Z is an action manga (with hints of comedy) staring Goku as an adult. Dragon Ball and Z were made into anime.
Dragon Ball GT is an anime series in which Goku, who is now around 50, is magically transformed back into a child and forced to travel the galaxy with his granddaughter or else the world will explode.