A UPC is the number with the bar code. The UPC is a Universal Product code which means the exact same number is used no matter where the product is being sold. You could take a product you buy from one store and bring it to another store and the UPC would be valid at both stores.
A SKU is a store specific number and would only be valid at the store where it is being sold it would not be universal.
A serial number is a sequential number that is assigned to a single item of a product. The serial number will only be for that one single item. It is there so you can track ownership and warranty information of that item. Serial numbers are common with electronic products.