UPC (Universal Product Code) is the standard for general products and is used by almost all barcode systems. Although, for books ISBN is more often used in addition. Amazon also its own system called ASIN (Amazon Standard Identification Number), although it’s not prevalent outside of Amazon.
SKU (Stock-keeping Unit) is another identifier that is often used, however, it is generally not universal for a particular product. For instance, each vendor for a particular product may assign its own SKU to that product. On the other hand, a product may have a MPN (Manufacturer Part Number, or a model number) that is assigned by the manufacture. While this number is unique, it is not very universal because each manufacturer uses it’s own numbering conventions.