ID3 is a metadata container most often used in conjunction with the MP3 audio file format. It allows information such as the title, artist, album, track number, and other information about the file to be stored in the file itself.
There are two unrelated versions of ID3: ID3v1 and ID3v2.
Although ID3 is sometimes referred to as a standard, the term applies only in the de facto sense, as no standardization body was involved in its creation nor has such an organization given it a formal approval status.
Read more about ID3: ID3v1, ID3v2, Editing ID3 Tags, Implementation in Non-mp3s and Alternatives