UPC (Universal Product Code)
EAN (International Article Number)
UPC or EAN codes are unique codes used to identify products such as an Album or a Single that you can buy in a physical or in a digital Store.
UPC are more common in America and Canada while the EAN are more common in Europe, but it is also used in other Countries as well. The codes are unique for each product. UPC can assume 1 trillion different values while the EAN can assume 10 trillions, so, the possibility of two different products having the same code is highly unlikely.
The decimals of these codes can be embedded in the final production CD or into the digital album (not required).
The barcode is also attached to the album in some way, either by your graphic designer at the time you are creating your physical album artwork, or after the fact by affixing labels.
CDs and Digital Albums only need one UPC barcode regardless of the number of tracks on the album. If you are going to be selling both a physical CD and a digital album, you will need a different barcode for each. The purpose of the barcode is to electronically help a retailer track sales and insure that the correct price is charged for the item being sold. Since a physical CD and a digital album are two different things, they need to be tracked differently.
More info about Bar Codes here.: http://www.adams1.com/upccode.html