For the most general of answers they degrade the sound quality. If you take a 40mb uncompressed song (wav) and throttle it down to 4mb (mp3) with bit rate and compression you get a loss in quality. In order to make an mp3 sound info gets thrown away by the amount of compression you use. Or are you looking for a more specific answer like how a certain type of compression effects the sound differently than another type of compression?