A photoresistor is a sensor whose resistance varies with light intensity.  Most decrease in resistance as the light intensity increases.  In a typical microcontroller application, this resistance must be converted to a voltage so that an A2D converter can measure it.  The easiest way to do this is with a voltage divider circuit.

A voltage divider is just two resistors in series connected between a voltage supply and ground.  If R1 is connected to the voltage supply and R2 is connected to ground then the voltage at the junction between the two resistors is:

If R1 is the photoresistor, the voltage will increase with increasing light intensity.  If R2 is the photoresistor, the voltage will decrease with increasing light intensity. 

Sample Photoresistor Circuit

Related Links:

Sensor Voltage Interface Basics Informational Article

Plan for week 4 (Jan 28th) of 407/507 Robotics Course