MOSFETs are Metal Oxide Semiconductor Field Effect Transistors. There are two broad categories of MOSFETs, Depletion MOSFETs and Enhancement MOSFETs. Depletion MOSFETs have a conductive channel between drain and source when no field is applied at the gate. Enhancement MOSFETs only have a conductive channel between the drain and the source when an appropriate voltage (electric field) is applied at the gate (between the gate and the source). Enhancement MOSFETs are easier to manufacture, so pretty much all development and manufacturing over the last few decades has been focused on E-MOSFETs. Depletion MOSFETs do have some purpose because their default case is to be turned on. Circuits that need a small amount current at start up but then need to be turned off could use a depletion MOSFET for that purpose.
The videos below provide a brief introduction to the characteristics of both depletion and enhancement MOSFETs and then look a little bit more in to using enhancement MOSFETs as switches
Depletion MOSFETs are “on” by default. To turn them off requires applying a voltage between gate and source. This video describes their characteristics and biasing. While the construction is quite different than the construction of JFETs, they behave in a very similar manner.
E-MOSFETs are widely used and can act especially well as switches. By default the are turned off and by applying the appropriate Gate-Source voltage, the MOSFET can be turned on. These two videos describe the structure and behaviour, of MOSFETs and includes a more in depth look at using MOSFETs sswitches