MP3, short for MPEG-1 Layer 3, is an audio compression format common on the Net these days. It can take larger audio recordings and shrink them down to a fraction of their size while losing little if any fidelity of the sound. It is a very common format with a lot of free players available online.
For free MP3 players, choose your architecture from the list below.
Here are some other useful links to get more information about MP3.