The CDDB is a huge user-generated database of information related to music CDs: album titles, songs, artist information, genre, and about 50 other attributes. The CDDB has been around since 1995, an early example of what is now referred to as Web services. Most computer CD players will automatically contact the CDDB database when you play a CD on your computer (if you're connected to the Internet), allowing you to see song titles while it's playing. The CDDB information comes in especially handy when you rip MP3s from CDs: the newly minted MP3s are tagged with the CDDB info.
Since the CDDB server is contacted each time a CD is played on a computer, the database contains lots of information about what's being played. Gracenote, the company that controls the CDDB, publishes top 10 lists for several genres each week. For the most part, people are listening to the same stuff that you see on MTV's TRL.
To store all that information in a useful way, each CD needs a unique identifier in the CDDB database. Before I started poking around a little, I assumed that each CD was burned with a unique key parcelled out by an agency like ISBNs for books. What actually happens is when the information from a CD is placed in the CDDB for the first time, the CD's TOC is stored in the database as the identifier for that CD. The TOC contains the lengths of all the tracks as well as the starting sectors of each track. That means that TOC is not necessarily unique...it's merely almost unique, causing the occasional misidentification.