uPVC (also known as PVCu) stands for Un-plasticised Poly Vinyl Chloride. This is the fancy way of saying a "Plastic based product". PVC is a popular material to be used in building and is a very flexible and superb material to be used.
When you see the "U", this means that when the plastic hasn't been softened during the creation process resulting in a much stronger material than standard PVCs.