The word vinyl brings to mind the vinyl record. It is a polymer chemical that is essentially a durable plastic used nowadays for construction purposes. In the raw form it is a liquid or solution. Upon addition of a polymerization factor, it turns solid by crosslinkage of all the polymers together. Adding particular chemicals known as pthalates helps give it flexibility.
Homeowners have a choice of vinyl windows and vinyl doors. The main advantage they have over other types of material is the relative low price and the low maintenance. Unlike wood, vinyl will not change shape over the seasons, nor will it split and crack from the constant dry and moist cycles of the climate.
However there are a few disadvantages. For one it does not impart the classic look of wood so may not fit the design aesthetics of the rest of the house. For another, vinyl doors and window frames are difficult to repair if they are damaged. The most convenient thing to do is to get a new one.
PVC is not as popular as wood or steel when it comes to doors.