Dragon Ball Z is all about The Z-Fighters of earth, protecting the planet from destruction at the hands of such as Vegeta, Raditz, Freiza, And Dr. Gero's Androids.
In Dragon Ball Z, Gokuu(introduced in DB), is now a married adult with a half-sayi-jin son named Gohan. The first fighter to arrive is Raditz. Followed By Vegeta And Nampa. Then, everyone ships off to planet Namek! Home Of The Green Guys!! To Collect Some Dragonballs. They end up leaving Gokuu on the planet, and heading back to earth. Gokuu beats the crap out of Frieza, the Head BadGuy, and heads back to earth, while Gohan, Piccolo, Kliln, and Kame try desperatley to save the earth from Garlic Jr.
Then, "A Mysterious Youth" comes from the future to tell Gokuu about 2 (which turns out to be 4 or 5) androids that will destroy everyone, everywhere, and about a heart disease that will kill him.
Three years pass, and the fight begins...
Gokuu Starts Beating the crap out of A Big Fat Android With A Girly Man's Voice, when the heart virus attacks him...
Androids Die, Cell comes to the future from the past. Gokuu is stuck in bed. Cell absorbs androids 17 & 18 to become Perfect Cell. Gokuu gets better, Cell fights Gokuu, gokuu gives up. Gokuu dies. Cell dies. Gohan goes to high-school. World martial-arts Tourney! Gokuu comes back to earth for one day. Vegeta goes bad and gets a big fat M on his head. Buu breaks out of his egg. Buu and Vegeta Fight.