You can't always go by the names things are sold under. The foam boards I bought for some fiberglass work were called "urethane foam" in the advertisement. They were actually polyisocyanurate foam, a.k.a. rigid insulation without any facing material. Another example: ever bought vinyl siding or tore a vinyl top off a car? Those are not "vinyl" but rather a polymer, most likely polyvinylchloride or plain old PVC.
As a follow-up to powderbill's organic chemistry lesson (thanks for that, powderbill!), see http://scifun.chem.wisc.edu/chemweek.../Polymers.html