Is it rubber? Is it plastic? Is it rigid? Is it flexible? The answer to all of those questions is - yes. Polyurethane covers a group of materials; plastic polymers produced by the combination (or synthesis) of di-isocyanates with polyols and a chain extender.
Thermoplastic Polyurethane (TPU) belongs to the Thermoplastic Elastomer (TPE) family. It was originally invented by Otto Bayer in 1937 and further developed during WWII as an alternative to rubber (which was difficult to source at the time). It can be formulated to produce different finished materials with an array of properties suitable for a wide range of applications.
In addition to seals, Polyurethane is the basis of products as diverse as varnish, foam mattresses, roller skate wheels and surfboards. There are literally hundreds of different types of polyurethanes, each made in a slightly different way to suit the demands of the final product.