The Dragon Balls, numbered 1 through 7, when collectively gathered, can be used to summon the eternal dragon who can grant wishes to the summoner. The Dragon Balls were originally created by the Namekians.
There are three different sets of Dragon Balls in total. There are the Earth Dragon Balls, Namek Dragon Balls, and Black-Star Dragon Balls. Each set of Dragon Balls can grant wishes, but their power and appearance differ slightly from the other balls. If the creator of a set of Dragon Balls dies, the Dragon Balls will turn to stone and become useless unless they are restored by another Namekian, or the creator is brought back to life.
The Earth Dragon Balls were made by the Namekian, Kami, when he traveled to the planet Earth. Once used, they are turned into stone, and cannot be used unless revived by Kami, or another Namekian for one year. If only one wish is granted, they can be located again in four months time. Once used, the dragon balls disperse across the planet and must be found again.
The Earth dragon balls summon the eternal dragon Shenron. He can grant one wish. When he was revived by Dende, he could grant two. He can only revive a person from death once.
The first Dragon Ball introduced was the "4 Star" ball. It was carried by Goku through much of the original Dragon Ball series. It was said that the "4 Star" ball held the spirit of Grampa Gohan, his adoptive Grandfather. These Dragon Balls could be found using the Dragon Radar, an invention made by Bulma Briefs of Capsule Corporation.
There are also seven dragon balls on the planet Namek, home of their origin. They are much larger than the Earth dragon balls. They summon the eternal dragon Porunga, who is capable of granting three wishes and has no limitations on reviving the dead, although he can only revive one person at a time. After Namek's destruction, the Namekian dragon balls followed the Namekians to their new home.
Not only does Porunga grant more wishes than Shenron, he can also be summoned every 130 days, as opposed to Shenron's full year.