The general differentiation of windows beyond style (double-hung, casement, sliding, etc.) is the glass or glazing and the framing material. There are three main types of frame materials, each addressing several lifespan aspects of the windows.
A natural product, it is said that the industry grows thousands of windows a year. A moderate insulator (R1 per inch), it requires some maintenance (stain or paint) to prevent rot from moisture build-up. It is warm to the touch in the coldest winters and room temperature in the summer.
A product of the plastics industry, it uses a nonrenewable petroleum source for extrusion. The final product is usually non-paintable, but does offer a lifetime free maintenance. Some radical climatic changes over time may stress the material to failure at the joints allowing water penetration, though it is rare with quality manufacturers.
A metal commonly used in cookware for its thermal conductivity and in airplanes for its strength to weight properties, it features lifetime free maintenance, but usually cannot be painted. Over the course of many years aluminum will oxidize leaving a dull pitted appearance. If not well insulated with a thermal break, it is very cold to the touch in winter and hot in summer.
Beyond the framing material one has a wide selection of glazing or glass combinations to fill the sash or window panels. Once the standard, single pane glass is usually available only as an option, whereas double pain insulated glass is the norm. But this will not last long with the triple pane windows on the market combined with high energy bills.
With Rising Utility Bills, triple glaze glass has become available. What material or substance placed between the panels differentiates dual glazed window systems. With the injection of a noble gas like argon, xenon, or krypton between the panels, the conductive resistance of the window improves. Suspending a polyester film covered with a highly reflective material like silver further improves window performance. Sometimes the second glass panel (interior facing) is spayed with a special coating, called low-e*, which acts as a reflective interface.
When ordering windows it is best to get the low-e coated windows or the suspended film type for greater seasonal comfort and energy savings.