LDR is a special kind of resistance, which varies according to the light falling on it.
An LDR is a kind of light sensor and is also called photoresistor. It is made up of photo resistive material Cadmium Sulphide.
LDR’s resistance varies with the strength of light falling on it. When it is kept in dark, its resistance becomes very high (few Mega Ohm). When light falls upon the LDR, Its resistance decreases to few Ohms.
LDR is used in automatic night lights to detect whether it is dark or light. It can be used to automatically turn on a light at a certain light level, e.g., a street light or garden light.