As mentioned, the ID3v1 and ID3v2 standards define a mechanism and syntax for storing metadata in the MP3 file itself. Many MP3 players, from car stereos that play data CDs loaded with MP3s to Ipods to Windows Media Player can read this data from the MP3 file itself.
The data is stored in the file itself, in a special part of the file. One section of the file contains the encoded sounds, the music, and another contains these tags. Most of my MP3 files contain both ID3v1 and ID3v2 tags, in addition to the encoded music, so that wherever I play the MP3, I can see the track name and artist name and so on.
There are several databases of tags out there on the interest. Music Brainz is one. FreeDB.Org (http://www.freedb.org/) is an older one. If you CD ripper supports accessing such databases, getting your MP3 files tagged is a snap - usually there is an option to query the database, and then you have a bunch of tags that match the CD you just ripped. Some media applications do this automatically for you.
I like Foobar2000 for seeing, and setting, all tags including arbitrarily named tags. I like MP3BookMarkHelper for setting/changing tags for a group of files, for example a multi-CD recording where I want to change the Artist tag for all the tracks.
The one thing I have found, especially with classical music, is that many people have different opinions about the "correct" way to tag MP3 files. For example, should the composer be the Artist? Should the Conductor be so enshrined? What about the orchestra or string quartet? A nice feature of ID3v2 is that there are many more tags defined in the standard, and it also supports the creation of custom tags, so you can tag your files in a way that you like.
Thanks for the great answer. Now if only I could convince iTunes to use this field for searches... – James A. Rosen – 2009-07-31T13:24:41.723