The light sensor sensor use a photoresistor which returns level of illumination in the form of a resistance value.  

The 4 pins are GROUND, Vcc (3-5VDC), DO (Digital Out), and AO (Analog Out).  

Using the AO pin a voltage proportional to the light intensity can be read from an Arduino analog input.  

Using the DO pin, the potentiometer adjusts the set point.  If the temperature exceeds the set point the DO level goes HIGH.

Board Size: Approx 3.2cm x 1.4cm/1.26 x 0.55 inch