The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of transistor used for amplifying or switching electronic signals.
The body (or substrate) of the MOSFET is often connected to the source terminal, making it a three-terminal device like other field-effect transistors. Because these two terminals are normally connected to each other (short-circuited) internally, only three terminals appear in electrical diagrams.
These are used in many scenarios where you want to convert voltages. On your motherboard for example to generate CPU Voltage, Memory Voltage, AGP Voltage etc. Mosfets are usually used in pairs. If you see six mosfets around your CPU socket you have three-phase power.