Tempered Glass is created when “normal” glass goes through a special heat treatment. After the treatment, the glass is four times stronger than normal glass of the same thickness. Tempered glass is considered safety glass because if it's broken it falls into very small pieces instead of dangerous shards.
One drawback to the tempering process is that it sometimes create a distortion in the glass. It also has to be ordered to the exact size needed, because once its tempered it cannot be cut or drilled.