Audio CDs consist of a sequence of music tracks in a digital format called CDDA-WAV. They were designed to be played by very simple consumer-electronics devices a few years before general-purpose computers developed enough raw speed and sound capability to decode them on the fly. Because of this, there is no provision in the format for even simple metainformation such as the album and track titles. But modern computer-hosted CD players want this information so the user can assemble and edit play lists.

Enter the Internet. There are (at least two) repositories that provide a mapping between a hash code computed from the track-length table on a CD and artist/album-title/track-title records. The original was cddb.org, but another site called freedb.org which is probably now more complete and widely used. Both sites rely on their users for the enormous task of keeping the database current as new CDs come out; freedb.org arose from a developer revolt after CDDB elected to take all that user-contributed information proprietary .

Queries to these services could have been implemented as a custom application protocol on top of TCP/IP, but that would have required steps such as getting a new TCP/IP port number assigned and fighting to get a hole for it punched through thousands of firewalls. Instead, the service is implemented over HTTP as a simple CGI query (as if the CD's hash code had been supplied by a user filling in a Web form).

This choice makes all the existing infrastructure of HTTP and Web-access libraries in various programming languages available to support programs for querying and updating this database. As a result, adding such support to a software CD player is nearly trivial, and effectively every software CD player knows how to use them.