Mosfets are an electrical switch instead of the mechanical switch. They are a useful addition to an AEG and can save your trigger switch contacts. Some of the newer computerized mosfet units allow you to fully customize your trigger experience. The way the mosfet units save your trigger contacts is to bypass with the higher voltage circuit running through the switch contacts. They just use the trigger contacts with a very low voltage circuit to activate the Mosfet electrical switch.
You can get simple Mosfet units, Active braking units, mosfet units with re settable fuses. They do versions with lipo protection and battery voltage detection. There are even a few now that are fully computerized mosfet controlled units.
On this page you will find information about the standard mosfets and their benefits.
This below information leaflet is from Airsoft International and APS who now install mosfets in all of their rifles.