Polyurethane isn't really a type of finish but rather an ingredient which, after sufficient drying, forms a plastic resin on the surface of the wood. Thanks to their superior qualities, polyurethane based lacquers and varnishes have become the most popular type of finish today. Shellac, for example, being a natural product, is completely overshadowed and is now used mostly in conjunction with wax by those who prefer natural alternatives.
http://www.kagemink.com/water-based-varnish/