CD's are something that everybody has had or has. But for such a universal product, we really don't know how they work...
Although the disc looks and feels flat, it actually is quite the contrary. On a CD is a ton of little pits. These pits have binary code in them. Then a Focusing laser goes over the pits, receives the code, and transfers it to a detection circuitry. The digital signal received is then converted to analog form by a D/A converter.
The Laser used is a semiconductor laser. There are also two prisisms, used in the system that help in the direction of light as well as multiple directions. The direction and the polarity of the laser light has to be manipulated in order to have a properly working system to play a CD.
As you can see, there is a lot more to a CD than just a spinning disc.