How is Toughened Glass different to Ordinary glass?

Ordinary glass will break when it is under a stress that it cannot withstand. This stress can be caused by sudden impact, increasing pressure upon certain points of the glass, or by cracks developing in the glass' surface.

So how is Toughened Glass different?

Any cracks in the surface of the glass serve to raise the stress in a particular part of the glass. But, cracks can only grow in areas of tensile stress, where adjacent molecules are already being pulled apart. This makes it easy for the cracks to spread. Because the surface of toughened glass is in compression, these small cracks have little effect upon the glass, because the internal compression forces act against their force upon the glass.