What quantities, how far, how much head will the pump need?
For a 20g container beside the tank with about 5 1/2 ft of head, I just use a Hagen 802 power head.
To go a distance and more head, will probably require a pump, the size to be determined by the first question above.
For MOST hobbyists, just about any kind of transfer pump will work, and, even if it's not meant for salt water, a thorough rinse afterwards in fresh water should suffice. A pump made to handle salt water will cost more, and would definitely be required for full time use.