How does liquid cooling work?

[milesdkenyon]

With the Corsair H60, does it need filling up or does it have liquid inside it and it doesn't need topping up? I'm going to be building my first rig so I may have to use the stock cooler for now.

Water cooling uses actual water and refills doesn't it so what's the difference?

 

[Bombular]

Comes prefilled with cooling liquid

 

[nvalhalla]

Closed systems like the H60 do not need to be refilled. They function the same as other water loops, but because they cannot be opened, they do not lose water over time.

 

[Vosgy]

The sealed units like the Corsair H60 come pre filled and normally you wouldn't need to add extra. However small amounts of the cooling liquid does slowly evaporate though the piping and seals. I believe some of the sealed ones can have the cooling liquid topped up but I'm not 100%.
Normally the cooling liquid wouldn't be actual water and if it is it would have to be pure water, because any imperfections in water makes it conductive, so if they leak they could destroy your electronics. The principle behind liquid cooling on the other hand is that air doesn't conduct heat very well from other sources like liquid and solids, Liquids conduct much better, so we move the heat away from the CPU or GPU using liquid that then gets replaced with cooler liquid and move the warm/hot liquid to a large radiator so the liquid can transfer the heat to air, the large radiator being necessary as air as mentioned earlier doesn't conduct well. This keeps weight off the CPU which is one of the down sides of large Air Coolers.