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Often confused with CD-Text, the online Compact Disc Database stores metadata on audio CD's.  The online database was created by Gracenote and is accessed by client applications, such as iTunes.  The client application sends a 'disc thumbprint' then the information in the database is downloaded and displayed in the program.  Sometimes, especially with burned discs, there are multiple entries in the database for that thumb print due to several users adding their personal 'mix cds' to the database.