These Balls were created by Kami-sama, just like the ones of Namek, his birth planet.
Whoever will gather the seven Balls, will be able to summon Dragon Shenlong and make a wish; then the Balls will scatter all over the planet Earth and will turn stone for a year. The existence of the Dragon Balls depends from the life of their creator, Kami-sama, so as the power of Shenlong.
When Dende becomes the new Kami on Earth, the Earthling Dragon Balls are modified by him so they can ask for two wishes (instead of only one), but however Shenlong can not make the same person revive more than once (as he couldn't before)!

On Namek, the Dragon Balls are passed from the Grand Elder to his successor, so that when he will die the Balls will survive in the hand of the new Grand Elder. As the Earthling Balls, also the power of the Namek ones depends from their creator. Once the Balls are gathered, Dragon Porunga can be summoned, and three wishes can be granted, but they have to be asked by using the Namek language.
Unlike Shenlong, Porunga is able to make the same person revive more than once.

These Balls are jealously guarded inside Kami Palace and, unlike the others, summon a red Dragon Shenlong; besides, once the wish is made, they scatter in the Universe and if they are not gathered and taken to the planet where they have been used within a year, this one will explode.