What is the difference between a normal CD-R disc and a CD-Audio (Music Only) disc?

CD-Audio (Audio only or Music only) discs have a hidden code or mark embedded in the subcode which identifies it is an audio disc. This means that the disc manufacturer has paid a royalty to the recording industry in order to produce the blank disc. Older stand-alone component CD recorders won't write to a disk that doesn't have this hidden code. However, computer CD recorders (or DVD recorders) don't scan for it, so it is not relavent.

As you might have guessed already, the difference in price between regular computer CD-Rs and audio CD-Rs is the royalty paid to the recording industry, not necessarily marketing or improved quality.