Dragon Ball and they successors Dragon Ball Z and GT was originally a manga of the same name (Draw by Akira Toriyama).
The manga was first shown on the weekly magazine Shonen Jump in Japan. Like many other successful manga, a TV series followed. After about 150 episodes. The Dragon Ball series stopped and a new series call Dragon Ball Z soon followed it. There are many reasons for this change, may be one of the strongest argument is the change of fighting styles.
The original Dragon Ball series was really fun to watch but may be Toriyama has ran out of idea. And as a result, Dragon Ball Z doesn't have much of a story at all. Many people calls it Dragon Ball nowdays. But nevertheless, Dragon Ball Z can still attracts many people with it high speed action, occasional humour and its what-next situation.