A dragon ball is a mystical object frequently used by the Z fighters. They are created by Namekians.
Earth Dragon Balls.
If one gathers all seven dragon ball's then they can summon the eternal dragon, Shenron and grant a single wish within the dragons power, however one must know the right password to summon the dragon or nothing will happen. Once they have been used they cannot be used for another year.
The Dragon Balls on Namek are very simalar to that of earth, however they are much larger in size, the dragon is named Porunga instead of Shenron, their Dragon Balls take only 180 days (a namekian year), you have to speak in Namekian to summon it and most importantly instead of having one wish you are granted three.
The Dragon Radar.
As a youth Bulma created a radar that tracks Dragon Balls, which have been of great use to the Z fighters as it would take a very long time to track them without it.