Liquid Cooling Basics
The purpose of any PC cooling system is to take heat away from your components and put it somewhere else.
With a traditional CPU air cooler, the heat is transferred from the CPU into a heat sink. A fan is actively pulling air past that heat sink, and as that air travels past the heat sink it draws the heat away from it. The air in your PC case is drawn out of the case and out the back by another fan or fans. As you can see, there's a lot of air movement:
With liquid cooling, coolant is used to transfer heat instead of air. Liquid coolant is pumped from a reservoir into a tube that transfers the coolant to where it's needed. The liquid cooling unit can be either in a separate unit outside of the PC case, or integrated within the PC case. In our diagram, the water-cooling unit is external.
The heat is transferred from the CPU into a "cooling block." The cooling block is simply a hollow heat sink with both an inlet and outlet for the liquid coolant. As liquid travels through this cooling block, it transfers heat along with it, and it works much more efficiently than air can.
The heated coolant is then pumped into a reservoir. From the reservoir it travels into a radiator where it is cooled, typically by a fan. It is then pumped out again to the cooling block, and the cycle begins anew.
Now that we have a good grasp of the basics of PC liquid cooling, what choices are available on the market?