This is where everything is assembled.
It is also where ancillary
information is added to the mp3 file, known as ID3
such as the composer, performing artist, and pictures (e.g., album
stored here. Unfortunately, there is little rigor with keeping these
tags standardized, composers names are entered in performing artist
fields, titles include the singers name, etc. Even so, there are
some databases available
on the internet with this information. Some programs can automatically
access these databases and then fill out your ID3 tags for you.
The ID3 tags were never
part of the original MP3 specification. Eric Kemp (aka, NamkraD)
in 1996 simply added a chunk of data at the end of
the MP3 file to contain more information about the file. Since then,
others have improved ID3 enabling to contain more data and placing
it just before the audio data, not at the end (version 2).
Of course, to use ID3 tags you need an MP3 player
that can read them (most current players have no problem with this).
Here are two diagrams comparing version 1.1 and