MP3

The name of the file extension and also the name of the type of
file for MPEG, audio layer 3. Layer 3 is one of three coding
schemes (layer 1, layer 2 and layer 3) for the compression of
audio signals. Layer 3 uses perceptual audio coding and
psychoacoustic compression to remove all superfluous
information (more specifically, the redundant and
irrelevant parts of a sound signal. The stuff the
human ear doesn't hear anyway). It also adds a MDCT
(Modified Discrete Cosine Transform) that implements
a filter bank, increasing the frequency resolution
18 times higher than that of layer 2.

The result in real terms is layer 3 shrinks the original
sound data from a CD (with a bit rate of 1411.2 kilobits
per one second of stereo music) by a factor of 12
(down to 112-128kbps) without sacrificing sound quality.

Because MP3 files are small, they can easily be transferred
across the Internet.