The pump is usually a centrifugal pump, much like you'd find in a car's cooling system. Some liquid-cooling pumps are submersible, and you can place them directly inside the coolant reservoir. Others need to be kept dry. If you're thinking about using a submersible pump, make sure its exterior doesn't get hot enough to heat all the fluid in the reservoir.
The pump is one of the most important parts of the system. Its flow rate determines how quickly the coolant moves through the tubes and blocks. If the water moves too quickly, it doesn't have time to absorb heat before moving on. If it moves too slowly, too much heat can build up around sensitive components. The complexity of the system affects the overall flow rate - the more resistance the fluid encounters within the blocks and radiator, the slower the overall flow rate.
The pump also has to be strong enough to move the liquid from the lowest point in the system to the highest. This is known as head pressure or vertical pressure, and it's especially important when liquid cooling tall server towers.
The system's radiator can be designed specifically for liquid-cooling systems, or it can be the heater core from a car. Heater cores dispel lots of heat - they provide the warm air for a car's heating system in the winter. However, they're not usually as attractive as radiators that are designed for use with a liquid-cooling system. See how everything connects together on the next page.