The light dependent resistor (LDR), or photo-resistor, is a variable resistor that depends on incident light (as opposed to the position of a shaft in a potentiometer).
The resistance decreases as light is increased. In the dark, resistances in the order of 
can be achieved. In very bright light, resistances are in the order of 100s of 
.
Used in a potential divider circuit, a simple light-to-voltage can be created.
The resistance is also dependent on temperature, so these devices are not ideally suited to accurate measurement or instrumentation.