Arduino and the Photoresistor

A photoresistor or Light Dependent Resistor (LDR) is basically a resistor that has a resistor that varies depending on how much light it is absorbing.
A photoresistor is made of a high resistance semiconductor.
The semiconductor absorbs photons and based on the quantity and frequency of the absorbed photons, it allows electronsto "jump" into the conduction band and complete a circuit.

This is a photoresistor:

And this is a detailed diagram of it's components:

This step-by-step guide will help you use a photoresistor with your Arduino.