How is the information physically stored?
From _The Compact Disc Handbook, 2nd edition_ by Ken Pohlmann, 1992 (ISBN 0-89579-300-8):
"Write-once media is manufactured similarly to conventional playback-only discs. As with regular CDs, they employ a polycarbonate substrate, a reflective layer, and a protective top layer. Sandwiched between the substrate and reflective layer, however, is a recording layer composed of an organic dye. .... Unlike regular CDs, a pre-grooved spiral track is used to guide the recording laser along the spiral track; this greatly simplifies recorder hardware design and ensures disc compatibility."
The construction of a CD-RW from top to bottom: label, scratch-resistant and/or printable coating, UV-cured lacquer, reflective layer (24K gold or a silver alloy, but this layer is only 50-100 nm thick), organic polymer dye, polycarbonate substrate (the clear plastic part).
A pressed CD has raised (lands) and lowered (pits) areas. A laser in the CD recorder creates marks in the disc's dye layer that have the same reflective properties. The pattern of pits and lands on the disc encodes the information and allows it to be retrieved on an audio or computer CD player.
Discs are written from the inside of the disc outward. For example the spiral track on a 74-minute disc makes 22,188 revolutions around the CD, with roughly 600 track revolutions per millimeter as you move outward from the lead-in (23mm from the center) to the outer edge at 58mm. If you "unwound" the spiral, it would be about 5700 meters long.
The construction of a CD-RW: label, scratch-resistant and/or printable coating, UV-cured lacquer, reflective layer (aluminum),upper dielectric layer, recording layer (phase change film, i.e. the part that changes form), lower dielectric layer, polycarbonate substrate (the clear plastic part).