An adjustable variable resistor is one of the more common generic types of variable resistors. It is used extensively because a knob can be easily attached to it; turning the knob changes the resistance value. In nearly any electronic device with a knob that can be turned, the knob is attached to an adjustable variable resistor.
A preset variable resistor is a very common type of variable resistor. Unlike the adjustable variable resistor, its mechanical design is not designed to be attached to a knob. Instead, a preset variable resistor's value is often adjusted with a small screwdriver.
These resistors are used for production tuning of electronic devices. For example, before a manufacturer ships a stereo to the store, the preset variable resistors are adjusted. A technician will adjust the variable resistors to improve the stereo's audio frequency response, improving its sound quality. These are often one-time adjustments.
Buttons and keyboards control the value of a digital adjustable variable resistor. It is, in effect, under computer control. When you press the volume control button of your television remote control, it sends a digital signal to a digital variable resistor network. There, the signal is decoded, and the requested resistor value is switched into the electric circuit.
Precise and accurate adjustments to resistors need to be made in electronic circuits to compensate for tolerances in electronic components. This is done with precision variable resistors. A precision variable resistor often has a very low maximum value, about 1 or 2 ohms. It can often be adjusted with resolutions below a thousandth of an ohm.