It refers to any transistor that uses an applied voltage (the field) as its mechanism to turn on or off. A basic transistor has three terminals. A FET's three terminals are usually called 'source', 'drain' and 'gate'. Applying a threshold voltage to the gate allows a current to flow from the source to the drain. a FET can be used as a switch simply by raising and lowering the gate voltage around its threshold value. FETs also operate as current amplifiers by increasing the gate voltage (above the threshold)
A MOSFET (metal oxide semiconductor field effect transistor) is the fundamental transistor behind most of todays electronics including computers. It has a metal contact for the gate and is separated from the bulk transistor (substrate) by an oxide layer, typically SiO2 (silicon dioxide). This makes sure no current flows through the gate.
Paul Speziale, B.S., Engineering Physics Grad Student, McMaster U.
'Nothing in this world is to be feared... only understood.'