Conventional rotating pumps need shafts to drive them.  To prevent liquid leakage at the point where the shaft enters the pump chamber, a seal is required.

Common forms of shaft seal include:

- stuffing box (or packed gland)
- lip seal (a close-fitting stationary ring around the shaft, usually of rubber)
- mechanical face seal (2 polished annular faces pressed against each other, one rotating, the other stationary.  Materials are commonly carbon and ceramic).

Stuffing boxes leak slightly, even when correctly adjusted.  Lip and mechanical face seals make a liquid-tight seal when new, but are subject to wear in normal service, and will eventually leak and need to be replaced.

Diaphragm pumps (and some other reciprocating types) are inherently liquid-tight, have no need of a shaft seal, and are classed as seal-less pumps.  They are often able to run dry without damage for extended periods.

In rotating pumps, the need for a shaft seal can be avoided with a magnetic drive ('magdrive') permitting torque and power to be transmitted across a stationary, liquid-tight membrane in the back of the pump casing.

Seal-less pumps are useful when leaks cannot be tolerated because the liquid is corrosive, aggressive, toxic, flammable, radioactive, volatile, penetrating, precious, or otherwise difficult or dangerous.