An LDR or light dependent resistor is also known as photo resistor, photocell, photoconductor.
An LDR is a component that has a (variable) resistance that changes with the light intensity that falls upon it.
It used in many electronic circuits, especially in alarms, switching devices, clocks, street lights, Toys and more.
It is used to turn ON or OFF a device according to the ambient light.
A typical light dependent resistor has a resistance in the darkness of 1MOhm, and in the brightness a resistance of a couple of KOhm.
This resistor works on the principle of photo conductivity. It is nothing but, when the light falls on its surface, then the material conductivity reduces and also the electrons in the valence band of the device are excited to the conduction band.
These photons in the incident light must have energy greater than the band gap of the semiconductor material.This makes the electrons to jump from the valence band to conduction.
These devices depend on the light, when light falls on the LDR then the resistance decreases, and in dark resistance increases. When a LDR is kept in the dark place, its resistance is high and, when the LDR is kept in the light its resistance will decrease.