Guides for product: Photo cell (CdS photoresistor)
CdS cells are little light sensors. As the squiggly face is exposed to more light, the resistance goes down. When it's light, the resistance is about ~1KΩ, when dark it goes up to ~10KΩ.
To use, connect one side of the photocell (either one, it's symmetric) to power (for example 5V) 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 it's light out and near ground when it's dark.
CdS photocells are not RoHS compliant, so if you need something equivalent but RoHS compliant, check out the phototransistor.
For a full tutorial with wiring diagrams, code examples, and project ideas, please read the photocell tutorial page!