The primary type of barcode used for retail packaging is a UPC-A barcode or simply, a UPC.
A UPC bar code also known as a Universal Product Code is a unique 12-digit code that allows wholesalers and retailers to track the sales and inventory of your product in their store (web and brick and mortar stores).
This allows them to manage inventory and to sell your products at their Point of Sales Systems (cash registers)
The bars on a barcode are merely a graphical representation of the numbers below the barcode.
For a complete tutorial of how barcodes work, please read our UPC Barcode Tutorial