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A FET is a Field Effect Transistor.

Typical FETs are nearly perfect switches. Next to no current flows when they are off, and they drop only minuscule voltage when they are on.

An N-FET (N-Type Feild Effect Transistor) is typically used to switch a load to ground. The gate of a Logic Level N-FET can be driven by a 3.3v or 5v logic output (GPIO) of a microcontroller.

A P-FET (P-Type Feild Effect Transistor) is used to switch a load to a positive voltage, like 3.3v or 5v or more. Typically you need to use a pull-up resistor and a small N-FET to create the gate drive for the P-FET.