The water pump forces cold coolant from the radiator into the engine, and hot coolant out of the engine and into the radiator.
The water pump is driven by a belt - either the timing belt or a serpentine or v-belt.
Broken belts will cause the vehicle to overheat quickly.
Water pumps can eventually develop leaks as the seal around the shaft becomes worn. Timing belt driven water pumps are usually replaced at the same time as the timing belt.
Acidic coolant can cause corrosion on the water pump impeller, reducing cooling efficiency.
Some water pumps use a plastic impeller that can crack -- causing the vehicle to overheat.