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LDR
LDR stands for Light Dependent Resistor. It's a resistor whose value will vary according to the intensity of the light it receives.
Cabling
To connect it to an analog input on an arduino, it must be connected to a high value resistor (typically 100K Ohms).
The value read will range between 750 and 1023 for a strong light source received and less than 100 for obscurity.