Simple definitions of these items are found below. It is important to understand that not all products will have a UPC or Manufacturer ID. Much of that depends on whether the manufacturer has gone to the effort of creating these.
UPC - The Universal Product Code (UPC) is an assigned number the manufacturer can obtain for a product through a registration process. This code is a completely unique identifier for that product. Many product will not have a UPC due to the amount of effort required. The UPC contains only numbers.
Manufacturer ID - This is typically the identifier assigned to a product by the manufacturer.
SKU - This is the identifier you will use for use in your store. You can create your own or reuse the UPC or Mfg. ID for the SKU.