MOSFETs

Metal-Oxide Semiconductor Field-Effect Transistors (MOSFETs), for the purposes of digital logic, are voltage-controlled switches.

They consist of a metal conductor, an oxide layer that acts as an insulator, and a semiconductor which has variable conduction. There is a drain, gate, and a source. The voltage between the gate and the source ($V_\text{gs}$) is what controls whether current passes between the drain and the source.

The NMOS transistor will close at a high voltage at the gate and close at a low voltage.

The dual to NMOS transistors is PMOS transistors which opens at high voltage and closes at low voltage.