Window glass was revolutionized in the 1970's. Insulated glass (two or more pieces of glass with a dead air space between) made its debut in the early 70's. Low E glass was introduced in 1979. The E stands for emissivity. Low E glass works by reflecting heat back to its source. It does this by utilizing an ultra thin metallic coating on or in the glass.
Among other things, sunlight contains visible light, UV light, and infrared (IR) light. Visible light enables us to see things. Ultraviolet light damages your skin, wood, fabrics, window treatments, furniture, draperies, floorings and causes colors to fade.
Infrared light is basically heat. Low E glass has the ability to allow visible light to pass while blocking certain amounts of UV light and IR light.