Reference. This is your own internal reference. It might be a number, or its reference from the storage location or its supplier, or anything that makes it unique and it is optional.
ISBN. This is the international identification for books and different editions of a book.
EAN-13 or JAN barcode. These are the numbers of the product's barcode, which are used worldwide in order to identify it. You can use either an EAN-13 or a JAN number.
An EAN-13 is the product's 13-digit international article number.
A JAN is specific to Japan, but is compatible with the international EAN.
Important: if your wish to sell your products on marketplaces (Amazon, eBay, etc.), your EAN-13 codes has to be entered.
UPC. A 12-digit barcode, and the first of its kind, more widely used in North America, UK, Australia and New Zealand.