When you go to sell your product at a retailer (i.e. Target, Costco, or Amazon.com), that retailer will have you fill out a product information form. On that form is where you will put your company and product details as well as your 12-digit UPC bar code number. The retailer then manually enters that information into their inventory management computer. This way, when they scan your bar code at the register, it calls up that form in their system and gives you credit for the sale. In other words, the bar code is simply a link between your product and the product information form you fill out for each store that sells your product.
A bar code is simply an inventory tracking tool that retailers use in their computer systems. For example, if you sell a t-shirt that comes in one color and 3 different sizes you would need to buy 3 bar codes. This way, if one of the sizes is selling faster than the others, the store will be able to tell instantly which one needs to be replenished without having to do a manual shelf count.
(excerpt from
Buy a Barcode | Register barcodes for retail products | Get a Barcode.)
GS1 is the offical provider of barcodes, aka UPC's.
If you are selling to Wal-Mart or Kroger, they only allow GS1 barcodes.
If you are selling to any other store, you can purchase them for cheaper from
Buy a Barcode | Register barcodes for retail products | Get a Barcode..
You can even order your barcode on stickers to place on your hangtag!!!
upc codes 
