What is SKU Number vs UPC Code

I am doing an inventory management project for a client.  I have been in this business for over 20 years but never had to deal with SKU or UPC numbers.  The client wants to scan items as they are received from vendors and scan items as they leave the store.  Very simple inventory management but I'm getting confused over SKU codes vs UPC codes.  My client requested a file from all of his vendors with the products that they handle.  When we received the files they had varying amounts of information but the one thing they all had in common was an 11 or in some cases 12 position number.  Some of the files had no documentation at all but several refer to that number as a UPC code.

My assumption is that this UPC code will match the barcoding that is on each product when it is scanned.  That way I can identify the product and update the inventory counts appropriately.  Is this corect?  I have not been able to import the files yet to determine if each of the UPC codes are unique.  If they aren't the whole scheme falls apart.

At times my client has referred to the numbers as SKU's.  I know that SKU is an industry term but I am not clear on what it is or how it is determined.

 Is one of these an industry standard number and the other developed for in-house purposes?

How are these numbers assigned or determined?

Is there anywhere I can look to learn more about this?