MOSFET stands for metal-oxide-semiconductor field-effect transistor. It is a field-effect transistor with a MOS structure. Typically, the MOSFET is a three-terminal device with gate (G), drain (D) and source (S) terminals. Current conduction between drain (D) and source (S) is controlled by a voltage applied to the gate (G) terminal. MOSFETs compare favorably with bipolar transistors in terms of relatively high-speed and low-loss operation. There are P type and N type by channel polarity, and there are enhancement type with normally off (gate voltage 0 V off) and depletion type with normally on (deactivated with gate voltage 0 V) by control method. Enhancement type is popular.
For an explanation of the MOSFET structure, please refer to the following document.
Structures and Characteristics: Power MOSFET Application Notes (PDF:484KB)