PCM
pulse code modulation

Sound is analog, and computers are digital. So for a computer (and that includes CD and DAT players) to deal with sound, the sound needs to be digitized. The most common technique for doing so is pulse code modulation. It's the native format used by WAV and AIFF files for representing sound. It's not well compressed, so sounds are often encoded further using a codec.

See also: ADPCM, AIFF, codec, WAV