A file format for storing audio data on a computer system. There are many file formats for storing audio files.
Today, one of the most popular audio format/codecs is MP3, which provides acceptable lossy compression for music files.
There are many newer audio formats and codecs, like OGG/Vorbis and WMA which claim to achieve improved compression and quality vs. MP3. While some improvements are unquestionable, format/codec advocates often exaggerate them.
Lossless compression of sound is not nearly as widely used outside of professional applications, as lossy compression can provide a much greater compression ratio, with nearly the same apparent quality.
There are many uncompressed data formats, most popular of them being WAV, which is a flexible file format designed to store multiple types of audio data. A similar approach can be found in the AIFF format.