Watercooling for the PC has been around for years in some form or another, for at least as long as HEXUS have been in business, with basic physics defining why you'd want to use it. That means for air cooling, to cope with increasing temperature in the heatsink you need to move the air across it faster. That's why thermostatically controlled fans in your PC will turn faster the hotter something gets. The increased rotational speed of a fan, depending on the heatsink it's moving air from, produces more noise due to the airflow and physical movement of the fan. It's also obvious that an aircooled heatsink is exchanging the heat in-place, rather than moving it somewhere more suitable before getting rid of it.
Alternatively water simply has more capacity to store heat than air, as a transport mechanism to move the heat from the source to where it'll be exchanged. So you pass water across your heatsink instead, obviously in a closed environment so the water can't escape, using that to carry the heat to an exchanger that can release it to the atmosphere, usually outside of your case. There's no spinning fan required depending on the size of the radiator, so there's less noise, the only needed moving parts in a watercooling loop being the pump moving the water around, and the water itself.
A regular aircooled heatsink has form factor requirements to comply to, limiting the size, weight and space it can occupy. Therefore watercooling, using blocks that fit into the same space as the heatsink and satisfy the same form factor requirements, solves most of the problems inherent to aircooling.
You can cope with more heat output, often from multiple heat-producing parts in your system using the same cooling loop. Everything is quieter due to less moving parts, even if you use fans on your radiator to exchange the heat. Lastly, the site you exchange the heat from can be placed optimally, often outside of your case. Overall, the only downside is cost, watercooling costing more financially and more in a time and maintenance sense than aircooling.
Basics out of the way, let's get specific.