There are some types of resistors that have a variable resistance value—the resistance can change. In a potentiometer, the resistance changes when a knob is turned. In a photoresistor, also called a light-dependent resistor (LDR), resistance changes when the amount of light falling on it increases or decreases.

According to how much you turn the shaft, the wiper connects to the resistor anywhere between pins 1 and 3. Turn it to the left, the resistance between pin 2 and pin 1 decreases, while the resistance between pin 2 and pin 3 increases. Turn the shaft to the right and the opposite happens.

Using a potentiometer, you can manually adjust the voltage in order to, for example, increase or lower the volume of a radio.

This diagram shows you how to build a simple circuit to dim an LED using a potentiometer. The extra resistor is there to ensure the LED is not damaged even if you lower the potentiometer resistance to zero.

LDR

In an LDR, most electrons are bound to atoms. This means the LDR cannot normally conduct electricity—it is an insulator. When light shines on the LDR, however, it supplies energy to free up its electrons, so the LDR now becomes a conductor. When an LDR forms part of a circuit, electricity can flow through it only when light is present.​​​​​​​

Voltage divider

If you connect two resistors with the same resistance to both the positive and negative sides of a battery, the voltage measured from a point between the two resistors will be half the battery voltage. If the resistors are unequal, the voltage is divided proportionately. The two resistors have created what is called a voltage divider. It is used to produce different voltage levels for different parts of a circuit.
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Using an LDR, you will get a voltage that depends on the amount of light. Using a thermistor, you will get a voltage that depends on the temperature. You could then connect it up to a circuit that automatically turns on an electric fan if the temperature rises above, say, 30°C. 

When it is light, however, the LDR has lower resistance. This makes the voltage flowing into the base of the transistor high enough to turn the transistor ON. The current now flows from the collector to the emitter of the transistor and on to the LED, lighting it up. By connecting it to another transistor that controls a buzzer, you can create a sound alert.