MOSFET

Despite what some people think, a MOSFET is a type of transistor (These people also clearly don't know what MOSFET stands for). Remember how a BJT is technically a voltage controlled device? While a MOSFET is a "better" voltage controlled device. Like a BJT there are three terminals: gate, source, and drain. Also like a bjt, there are two classes of devices: N-Channel and P-Channel (technically there are also enhancement mode and depletion mode MOSFETs, but no one uses depletion mode).

One big difference between a MOSFET and a BJT, is that the gate is fully isolated from the drain and the source. That is, there is a complete insulation layer between. This means that if there is a DC voltage signal on the gate, no current will flow into it. This is advantageous because we are not wasting any power turning on the MOSFET (At least using a truly constant DC signal).

With an n-channel MOSFET, applying a voltage to the gate with respect to the source will lead to a drain to source current. How much current depends on the gate to source voltage. A good way to quantify this is by looking at the IV curves found in MOSFET datasheets. You can look up a common N-Channel MOSFET like the 2N7000 and look at the IV graphs given in the datasheet (I'm not going to put it here because that would be plagiarism or something).

As you can see, at different gate to source voltages, the response is initially approximately linear, so looks like a resistor. Then it flattens out. The key to switching with a MOSFET is making sure you keep it in the linear region. If you do this, it is basically a voltage controlled switch with a somewhat known on resistance.

The drain to source on resistance is so important when using MOSFETs for switching that it is often given as its own specification. Of course, it is dependent on gate to source voltage and other stuff, and your MOSFET must be in its resistive region for it to apply. But, generally, if your on resistance is low, your MOSFET is a better switch.