Compact Disc

Compact Discs are an optical medium which store digital data and have amazingly been around since 1982. Over 200 billion have been sold worldwide and if I'm not mistaken that's over 33 for every person on the planet.
CD's are made from 5 layers, the first is a polycarbonate which is a type of thermoplastic polymer this is the part of the disc which contains the data. It is imprinted on the disc by a series of "pits" and "lands" as they are known.
These pits and lands are read by laser (light amplification by stimulated emission of radiation) the slight difference in distance that the laser travels before being reflected causes a difference in the intensity of the beam and it is this difference in intensity which allows the data to be read.
The reason a CD has the shiny reflective surface shown clearly in this photograph is because they have a very thin layer of aluminium (for the silver discs, the yellow discs have a layer of gold) placed within them.