SKU stands for Stock Keeping Unit. This is mainly used for your own administration. You can pick any number / word combination. You should pick a SKU on which you can easily remember the product.
These are universal product identifiers used in stores all over the world. You are required to provide the product UPC / EAN to list on Amazon.
UPC stand for Universal Product Code. It is the 12 digit barcode that is used in US stores.
In Europe, they use a different barcode: EAN. The most commonly used EAN standard is the thirteen-digit EAN-13 while the 8 digit EAN barcodes (EAN-8) are used less often on small packages.
You can obtain the UPC codes from your suppliers if they have already obtained the codes for their products. If you do not have barcodes, officially you need to buy UPC codes from GS1.
You can also purchase barcodes from third-party Sellers such as barcodestalk.com but ensure these are not already assigned to a product. Amazon will not allow duplicate barcodes to be uploaded.
Don't have barcodes for your products? You may be able to apply for an exemption to list without product IDs on Amazon.
Amazon Standard Identification Numbers (ASINs) are unique blocks of 10 letters and/or numbers that identify items. ASIN is Amazon’s own SKU they use to manage and organise all products. You can find the ASIN in the URL of a product on Amazon.
The ASIN can be found right after the /dp/ part in the link. In this case it is: B07DNBX1C4.
“Fulfillment Network Stock Keeping Unit” refers to the Amazon's own barcodes used for labeling your products to track them in their warehouse. FNSKU labels need to be placed on every product unit that goes into the FBA warehouse unless you opt to use "manufacturer tracked" FBA, which uses the UPC / EAN barcode.