From BBC NEWS:

The music industry’s methods of tracking down suspected music pirates have been revealed for the first time. Using digital fingerprints, or “hashes”, investigators say they can tell if an MP3 file was downloaded from an unauthorised service. The industry also tracks “metadata” tags, which provide hidden clues about how files were created. The details were given by the Recording Industry Association of America (RIAA) in a legal case against a suspected pirate.