Like most computer
backing store, a CD-ROM is just a huge load of 0s and 1s. The data is read with a
laser in a
CD-ROM drive. Here is a diagram.
\ \
\ \ <- laser
\ \
///\/////\\\\\////\\\\\/////\\\////\\\////\\\///
------------------------------------------------ CD-ROM
The laser iterates over the surface of the
CD-ROM, shining on those dashes. They represent little reflective bits on the surface of the CD. The
CD-ROM drive has sensors to catch this reflected light. If a certain sensor is activated, it's a
1. Otherwise, it's a
0. Nice and simple - the dashes are just reflecting the laser in the appropriate direction.
The reason CD-ROMs are read-only is because the process of "burning" these reflective surfaces could only take place once on a CD. Now, there are a hundred or so layers of the reflective material on CD-RWs, so one layer can be burnt totally away and a series of data burnt into the next layer. Cool.