A variable resistor is a potentiometer with only two connecting wires instead of three.
However, although the actual component is the same, it does a very different job. The pot allows us to control the potential
passed through a circuit. The variable resistance lets us adjust the resistance between two points in a circuit.
A variable resistance is useful when we don't know in advance what resistor
value will be required in a circuit. By using pots as an adjustable resistor we can set the right value once
the circuit is working. Controls like this are often called 'presets' because they are set by the manufacturer before
the circuit is sent to the customer. They're usually hidden away inside the case of the equipment, away from the
fingers of the users!