https://bcae1.com/tranfet.htm
https://www.bcae1.com/trnsistr.htm
The first line is for the Mosfet, the second line is for a Bi-Polar transistor.
It the most basic form they are an amplifier. They typically have three pins - a source, a drain, and a gate. When there is voltage (not current) on the gate then the mosfet "turns on" and connects the drain to the source allowing current to flow through the transistor. The gate requires virtually no power to turn on and the source to drain connection is capable of switching high current.
There are two main types - N channel and P channel. The N channel turns on when + voltage is applied to the gate. The P channel turns on when ground is applied to the gate. N channel mosfets switch a - signal on/off and P channel switch a + signal on/off.
Another big difference between a mosfet and a bipolar transistor is that you can't control the current output of a mosfet like you can a normal transistor. A normal transistor has a gain and the output is a function of the input. Say the gain is 50 and you put 1A in to a normal transistor you will get 50A out. A mosfet is either on or off so to control current you typically use PWM (turn it on and off really quckly in a ratio that will determine total power output).
Another characteristic of mosfets is their ability to turn on and off extremely quickly. Where an electrmagnetic relay turns on in 5-20 milliseconds a Mosfet will turn on in 1 microsecond.
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Kevin Pierson