What is the Difference Between PVC and PVCU?

PVC is the common popular term used to describe any plastic made from the polyvinyl-chloride. The "U" part of PVCU denotes that it is Un-plasticised and so has not had a plasticiser added to it to make it soft (eg: for tarpaulins) - so, more correctly, any rigid PVC product should be termed UPVC or PVCU. Most people, however, just call it PVC.