What is a Barcode?
A barcode is an image used to represent a small amount of information which can then be easily read by a barcode scanner or cell phone app. Barcodes come in many shapes and forms, however they are mostly seen on retail products for pulling up price and product information at the checkout. In this case the barcode is simply a 12 or 13 digit number encoded as an image.
Look here for CD barcodes, DVD barcodes, ISBN book barcodes and ISSN magazine barcodes.