Glass is tempered by heating it to the top of the annealing range and then rapidly cooling the glass in order to create surface and edge compression ("controlled" stress) in the glass and increase the tensile strength.
Typical window (float) glass that has been tempered is four to five times stronger than non-tempered glass.
Glass must be cut to shape before tempering as it cannot be cut afterwards. The glass must also be free from bubbles or other interior defects. Cutting, breaking, or chipping tempered glass will cause the glass to break into many small pieces.