UPC-A and EAN-13
UPC-A has 12 digits. Add a leading zero and you get an EAN-13 with exactly the same bars, which is why checkouts everywhere read both. The first digit is the number system: 0, 1, 6, 7 and 8 for regular products, 2 for random-weight items, 3 for drugs and health products, and 5 for coupons.
For retail or Amazon listings, use a number licensed from GS1 US or your national GS1 office.