Adjustable resistors, also known as potentiometers, allow a degree of control over the resistance within a circuit. This gives them an advantage over other types of fixed resistor, depending on your needs.
They are composed of a fixed resistive element, and a slider that is able to pass along the element. This is what changes the resistance of the circuit, and it is usually controlled externally.
Adjustable resistors are most commonly used for volume control in TV and radio. They can also be used as part of a light dimmer, to indirectly affect the level of light.
Adjustable resistors largely vary based on material