Lossy

Lossy compression is a form of compression that significantly reduce multimedia file size by throwing away information imperceptible to humans.

Human audio perception is not perfect. Lossy compression takes advantage of this characteristics. By selective discarding, much unnecessary information is thrown away. The amount of information discarded is usually adjustable, giving a compromise between small size bad quality and high quality large size.

The downside to this is that waveform reconstructed from compressed information will never exactly match the original waveform.