Throughout Dragonball and Dragonball Z the eternal dragons were used rather often. Not only did they have the power to revive the dead, but they even had the power to revive an entire planet. In total 2 dragons exist, Shenron and Porunga.

Shenron; resides on Earth, he was once created by Kami [the other half of Piccolo Daimao] and has certain restrictions compared to the Porunga. 
Restrictions;
1. You cannot revive someone more than once.
2. You cannot revive someone that died of natural causes.
3. You cannot have somebody killed, or any destruction at all.
4. You cannot wish for more wishes.
5. You cannot wish to be more powerful.

Porunga; resides on Namek and later New Namek, he was once created by Saichouro (the Namek elder) and and is noticably more powerful then Shenron.
Restrictions;
1. You cannot revive someone that died of natural causes.
2. You cannot have somebody killed, or any destruction at all.
3. You cannot wish for more wishes.
4. You cannot wish to be more powerful.
5. You can only revive one person at a time.


The Most noticable difference between the two dragons is the fact that Porunga can rivive someone as many times as you may need, Shenron can only revive someone once.