How Analog-to-Digital Converter (ADC) Works
If you're up for a good read, head on over to Hardware Secrets to check their latest 'how stuff works' article. Today they cover mostly the audio side of analog-digital conversion, and manage keep the explanations fairly easy to understand.
"Signals in the real world are analog: light, sound, you name it. So, real-world signals must be converted into digital, using a circuit called ADC (Analog-to-Digital Converter), before they can be manipulated by digital equipment. In this tutorial we will give an in-depth explanation about analog-to-digital conversion yet keeping a very easy to follow language."