In simple terms, the purpose of a window is to admit light while protecting a building interior from the elements. A stained glass window is made of pieces of colored glass, held together in lead channels. In this form a window can control light, create privacy, and provide a decorative enhancement to the architecture.

The "stain" in stained glass is not something added on top of clear glass, but is color included in the glass manufacturing process. Windows can be made from pieces of clear or textured glass held together with lead, in which case the window is called simply leaded glass.

A leaded stained glass window is made of colored glass, approximately one eighth of an inch thick, with each piece held in grooved strips of lead, called cames. These are soldered at the joints, and the whole is enclosed within a frame and fixed into the window opening.

Glass is made by the high temperature fusion of silica or sand, alkali potash or soda, and other bases such as lead oxide. The various metallic oxides added during manufacture are what create the colors in stained glass.

Stained glass comes alive in light, ever changing, taking on elusive qualities of brilliance and movement. In the hands of master designers and craftsmen, a stained glass window becomes a living work of art.
Reference:: The Story of Stained Glass. Published by The Stained Glass Association of America.