Dragon Ball Z is the name of the long-running successor to the popular Japanese animation, Dragon Ball. Specifically, it follows the adventures of the adult Goku and company as they defend the Earth against assorted villains. The distinction here is that while the Manga Dragon Ball follows Goku through childhood into adulthood, Dragon Ball Z is the designation given to the animation series to distinguish his adult adventures from the adventures he had as a child, which holds the original name Dragon Ball. The distinction is significant in that Goku's earlier adventures were much more comedy oriented, while as an adult the series took on a more dramatic and decidedly serious tone.
Dragon Ball Z was adapted from the manga Dragon Ball created by Akira Toriyama. The main character is Goku, an ultra-strong, yet simpleminded and kindly character. After a visit from his previously unknown brother, he discovers that he is an alien that was sent to Earth to destroy it. When he informs his brother Raditz he no intention of doing this, Goku and his friends become the erstwhile defenders of the Earth from the oncoming invasion by more of his kind.
As the series progresses, Goku, his son and their companions age, get immensely stronger, and fight increasingly powerful and sinister villains. Many of the main characters die, are resurrected, get married and/or have children. The series has had a dramatic progression throughout its entire run.
Many important facts were introduced into the series that add a different story arc to Dragon Ball Z from its predecessor:
Goku is an alien from the planet Vegita. This is the source of his previously unexplained abilities, and why he has a tail.
Goku's enemy from the first series, Piccolo is an alien from the planet Namek. He is the product of Kami (guardian of Earth) splitting himself in half to purge the evil from himself.
The origin of the dragon balls as the product of Kami.
The series can be logically broken into sagas based on the current enemy:
The Frieza saga
The Cell saga
The Buu saga
GT Saga Begins
Takes part after they have killed buu and all that other good stuff which i will probably think bout changing later.
Note: in case anyone gets the desire to add info about the influence of Journey to the West or Superman on Dragon Ball Z, do the smart thing and note it under a Dragon Ball Manga wiki. Dragon Ball Z is a subtopic to DB, and DB should be referenced for info like that. Same thing about Toriyama--he's done enough stuff to get his own page. thnx