What, exactly, is a MOSFET? I've built several computers, and am fairly conversant with names, but I as I was recently planning a new watercooled build, someone recommended MOSFET coolers, and I did a doubletake, because while I've heard the word, I don't actually know what a MOSFET looks like, what it does, etc. Thanks!

The metal–oxide–semiconductor field-effect transistor ( MOSFET , MOS-FET , or MOS FET ) is a device used to amplify or switch electronic signals.

They are located near your CPU socket, if your motherboard has the heatpile cooling from the manufacture like my motherboard has it cools the northbridge and the MOSFET's. Some cool the southbridge chip along with the northbridge chip and MOSFET's.

If your board has this type of cooling and you remove it to watercool the northbridge you would need to either watercool the MOSFET's or add some other type of cooling for them. They do get hot. If your just going to watercool the CPU or maybe the CPU and your video card usually you don't need to worry about it.

So it depends on what you plan to watercool and what motherboard your using

MOSFET is short for metal oxide semiconductor field-effect transistor. It is normally cooled by air flow generated by a heatsink fan. When you install watercooling, the airflow is not present, therefore you need a separate fan to cool it. Can't tell you exactly what it looks like, as they differ in design from motherboard to motherboard. It might be shown in the manual that came with the motherboard.

So they're under the heatsink near the CPU socket, or are they the voltage regulators next to them?

They are part of the voltage regulator, usually under the heatsinks near the CPU socket.

Gotcha, thanks!