Compact Discs (CDs) and Digital Video Discs (DVDs)
A compact disc contains a metal layer hundreds of millions of tiny bumps arrange in a spiral track, much like the grooves on an old vinyl record.
When you play a compact disc, the disc is rotated and laser light is reflected from the bumps. The reflected signals are turned into electrical pulses and decoded to produce sound. Digital video discs are much the same idea, also they are able to store the much larger amount of information needed to create full colour television pictures with associated sound.