Mp3
The MPEG-1 Audio Layer 3 format is a commonly-used lossy music format.
Format
MP3 files consist of a well defined header, music data, and optional metadata which may be at the beginning or end of the file, depending on the version of ID3 used.
Metadata
MP3 files may contain ID3 meta-information, including information about the software used to create the MP3.
Steganography
Using programs such as MP3Stego, MP3 files may be used as a base for steganographic content.