Photo Resistors or CdS cells (Cadmium Sulfide cells) are light sensitive devices. These devices behave like a resistor and typically decrease in resistance as it is exposed to light. They are also refered to as light dependant resistors (LDR).
This type of sensor is typically much slower than a photodiode at detecting changes in light, but it's benefit is in it's low cost and it requires only one resistor rather than a complicated photodiode amplifier. Photo resistors usually have a response in milliseconds so for normal day/night detection it works very well.
To wire a photo resistor you simply need to attach it in series with a resistor and hook the chain up to a voltage rail and a ground rail. Then you measure at the center node as if it were a voltage divider using the formula
If the photo resistor is R1 then the voltage will increase with light intensity
If the photo resistor is R2 then the voltage will decrease with light intensity