MP3 - (MPEG Layer-3 Audio, MPEG-1 Audio Layer 3) - MPEG-1 Audio Layer 3, more commonly referred to as MP3, is a popular audio encoding format. It is the primary audio format used in iPODs. It requires that files being downloaded be sent in their entirety before they can be played. The opposite of this is steaming, which starts playing before it's completely downloaded.