A SKU - Stock Keeping Unit is a product you purchase, produce and or sell. The SKU has to be unique, which means that two different SKUs have a certain attribute distinguishing one from the other e.g. package size. The same product in different package sizes are different SKUs. E.g. A 1 kg bag of sugar and a 2 kg bag of sugar, are two different SKUs.
A SKU has a SKU code or number for each particular product in a specific package size.
SKU is the term commonly used in warehouse management systems and inventory software, and therefore it is also the term used in tracezilla - an ERP system designed for food companies.