Vinyl windows are composed of extruded Polyvinyl-Chloride frames set with pre-glazed panes of glass. They have gained popularity in the last couple decades as they are often an efficient and economic alternative to aluminum and wood windows, their most common competitors. Also they have proven to be relatively low maintenance, have relatively good insulating value, and resist rot/corrosion/termites etc rather well.
Some negative aspects of vinyl windows are yellowing of white tinted windows, washing out of color tinted vinyl, and splitting or cracking in hot or cold temperatures. However, many newer high-end or ‘premium’ vinyl windows within the last 5-10 years have better formulations which resist these problems much better.