A very good web site to learn about laser is How stuff works:
howstuffworks You will learn all you want to know about lasers.

In our ScienceLine database we have an answer to how CDs work. I have posted it here. Together both should help you understand lasers and CDs.

How do CDs work? Electronics are often times based in the "binary" language, meaning that everything is presented by 0's and 1's. It is the easiest scenario because you only need to find out if something is "on" or "off." CD's are composed of many concentric grooves, much like the old vinyl records. Each groove of a CD is like a long trench. However, the trench is not uniformly deep, but rather there are two different heights. Imagine the bottom of the trench being a race track with a set of irregularly spaced hurdles. Now, a laser shines into the trench, hits the bottom, reflects back out, and then hits a detector that records the event.

Depending on if the laser hits the bottom of the trench or hits the top of a hurdle, a different signal is observed by the detector. Since we have only two levels, there are two different output signals. One signal is assigned a value of "0" and the other a value of "1" - this is how binary information is stored on a disc! The binary signal is then decoded by the CD player to be turned into the music that you hear.