as ever Google is your matey
sku is usually a number which indicates a specific product. its usually found in point of sale systems used in retail. its a way of mapping a physical product to a specific code. so a SKU usually refers to a specific product on the shelves. think of it as a way of translating the barcode (or the manufacturers product no) into an internal(local) product ID.
one of the problems often encountered is that a manufacturer will print a barcode (eg EAN or Code 39), which the POS should identify as an internal product. but retailers don't always register each barcode so sometimes the barcode isn't on the system
another issue is where the retailer may do a special on a specific product, or the may sell different multiples of a product (eg there is a one off price... the barcode, and there is a case price which is not the same as the number in the case times the one off price)
years ago a local retailer to me was selling 4 packs of 10xCD's, however the 4 pack was shrink wrapped and the till operator rang through the 4 pack (40CDR's) at the one pack (10CDR price). the problem.. the retailer hadn't issued a new barcode for the product, or if they had they hadn't made it clear on the package when scanned. at the time it was a very very good deal
I'd rather be riding on the Tiger 800 or the Norton