To the casual observer, there's little difference between PVC pipe and UPVC pipe. Both are plastic pipe used extensively in building. Beyond the superficial similarities, the two types of pipe are manufactured differently and thus have different properties and slightly different applications in building and other industrial processes and most repair-work exposure to plastic pipe is to PVC rather than UPVC.
PVC and UPVC are largely made of the same material. Polyvinylchloride is a polymer that can be heated and moulded to create very hard, strong compounds such as piping. Because of its rigid properties once it's formed, manufacturers frequently blend additional plasticising polymers into PVC. These polymers make PVC pipe more bendable and, generally, easier to work with than if it remains unplasticized. Those plasticising agents are left out when UPVC is manufactured---the name is short for unplasticized polyvinylchloride---which is nearly as rigid as cast iron pipe.
For installation purposes, PVC and UPVC pipe are generally handled in the same fashion. Both can be easily cut with plastic-cutting hacksaw blades or power tools designed to cut PVC pipe and both are joined using gluing compounds rather than through soldering. Because UPVC pipe doesn't contain the plasticising polymers that make PVC slightly flexible, it must be cut perfectly to size because it doesn't allow for give.
PVC pipe is used as a replacement for copper and aluminium piping on non-potable water, replacing metal piping in waste lines, irrigation systems and pool circulation systems. Because it resists corrosion and degradation from biological sources, it's a durable product to use in plumbing systems. It's easily cut and its joints don't require soldering, fastening with glue instead, and offers a little amount of give when pipes aren't sized perfectly, so PVC pipe is frequently chosen by handymen as an easier-to-use alternative to metal piping.
The use of UPVC isn't quite as widespread in plumbing in America, though its durability has helped it to become the material of choice for plumbing sewage lines, replacing cast-iron pipe. It's also frequently used in manufacturing exterior drainage systems such as rain gutter downspouts.
The only type of plastic pipe that should be used for transmission of drinking water is cPVC pipe.