Actually, the Earth Dragon Balls will summon the Earth dragon, Shenron, who will then grant anyone who asks one wish. A typical tactic (or attempt) in Dragonball and Dragonball Z is for someone else to shout out a wish before the summoner can (usually from hiding). For example, in the Dragonball series, once Oolong rushes onto the scene in a singular act of bravery and wishes for panties. No kidding. Yet another reason Dragonball is better than Dragonball Z.
The Namekian balls, however, summon a different and more powerful dragon who grants three wishes. He also lets you wish for the same thing more than once, though there may be a limit. (Curse my short-term memory.)
Additionally, the Dragonballs are created by the planet's guardian, a Namek alien, and will be rendered inert when the guardian dies. This, too, has happened more than once.
The final catch is that when the seven Dragonballs are used, they are scattered about the world and rendered inert for one year. They don't even show up on Bulma's Dragon Radar, which makes picking wishes tougher for the heroes. (The villains always stick with simple things like immortality, while the heros continually have to bring people back to life, mostly Goku and Krillin.)