[Guyute]

Here it is in a nutshell:
when a laser hits the copy layer in the CD, it leaves a hole (like a lens will focus sunlight and burn a hole in paper). This creates pits in the copy layer.
would look like this if viewed side-on:--_--_---_-___--__---_-

+R means the info is the unburned layer;
-R means the info is the burned pit. Or vice versa? But that is what the technology does.