An ASIN (Amazon Standard Identification Number) is a unique product identifier generated by Amazon when your title is published.
A content provider SKU (Stock Keeping Unit) is an identifier for a product that is unique within your title catalog. You assign a SKU when you create a title and can use this SKU to search in your title catalog. You can use your UPC as your SKU but your UPC is a different identifier than your SKU.
Universal Product Code (UPC) is a 12-digit code that is a unique identifier and is required for sale in the United States. On the Distribution page you can choose to use your own UPC or have a UPC generated for you.