The term stained glass can refer to colored glass as a material or to works produced from it.
As a material, stained glass is glass that has been colored by adding metallic salts during its manufacture. Painted details and yellow stain are often used to enhance the design.
Stained glass, as an art and a craft, requires the artistic skill to conceive an appropriate and workable design, and the engineering skills to assemble the piece. A window must fit snugly into the space for which it is made, must resist wind and rain, and also, especially in the larger windows, must support its own weight. Many large windows have withstood the test of time and remained substantially intact since the late Middle Ages. In Western Europe they constitute the major form of pictorial art to have survived. In this context, the purpose of a stained glass window is not to allow those within a building to see the world outside or even primarily to admit light but rather to control it. For this reason stained glass windows have been described as 'illuminated wall decorations'.
Source: Wikipedia
Beautiful and interesting stained-glass artworks are at the corridors of an exhibit hall at the Basilica of St. Ignatius.