The term stained glass generally refers to glass that has either been painted and fired or coloured by adding metallic salts during its manufacture and often both. The latter process is exemplified by, for example, the use of copper to produce green or blue glass or gold oxides to produce reds and oranges.

Stained glass is an art and a craft that requires the artistic skill to conceive the design, and the engineering skills necessary to assemble the piece so that it is capable of supporting its own weight and surviving the elements.

The process used in the 12th century (described below) has changed remarkably little even in modern times: The molten glass was annealed in a furnace to produce sheets of coloured glass. This so-called 'pot-metal' glass was sometimes rather dark and, to overcome this, 'flashed glass' was made by dipping a lump of white glass on a blowpipe into a pot of red glass and then blowing. This provided sheets of glas with the thin layer of colour. This could then be made bicoloured by grinding off some parts of the colour. The coloured glass was cut into different shapes with a 'grozing iron' and laid out on a table over the original design so that details of the drawing could be seen through it and painted with the oxide pigment on the surface. The pieces were then fired in a kiln.

The oxides permanently fused with the glass to produce the painting, this is the derivation of the term "stained glass". The pieces were then re-assembled with strips of shaped lead, the glass being slotted into the grooves on each side. The pieces were then soldered together, and an oily cement rubbed into the joints to make them watertight, and installed in a frame to create a window.

Stained glass made by Stanisław Wyspiański in Franciscan Church in KrakówCopper foil is now sometimes used instead of lead. For further technical details, see Lead came and copper foil glasswork.

Modern coloured glasses are available in varied textures—smooth, wavy, rippled, hammered, pebbled, or very rough. Stained glass is sold by weight and by square foot in sheets, usually about 3' x 4'.

Although described as 'windows' the purpose of stained glass is not to allow those within a building to see out or even to primarily to admit light but rather to control it. As such stained glass windows have been described as 'illuminated wall decorations'. The glass provides visual clues to the purpose of the building and, in a church, tells the Christian story.