Adafruit Photo cell (CdS Photoresistor)is a CdS cell which are little light sensors. As it is exposed to more light, the resistance goes down. When its light, the resistance is about 5-10KΩ, when dark it goes up to 200KΩ.
To use, connect one side of the photo cell to power and the other side to your microcontroller's analog input pin. Then connect a 10K pull-down resistor from that analog pin to ground. The voltage on the pin will be 2.5V or higher when its light out and near ground when its dark.
Adafruit Website: http://www.adafruit.com/products/161