How is bulletproof glass made bulletproof?

Bulletproof glass is a type of glass that is resistant to being struck by bullets. The correct term for "bulletproof" glass is bullet resistant. Basically it is layering of a polycarbonate material (usually consisting of products such as Armormax, Makroclear, Cyrolon, Lexan or Tuffak) between pieces of ordinary glass in a process called lamination. This process creates a glass-like material that is thicker than normal glass. The strength and durability of bullet-resistant glass depends on how it is made and the thickness of the final product. 

The use of plastic in the laminate provides impact-resistance. A bullet fired at a sheet of bullet-resistant glass will pierce the outside layer of the glass, but the layered polycarbonate-glass material is able to absorb the bullet's energy.  The ability of bullet-resistant glass to stop a bullet is determined by the thickness of the glass. A rifle bullet will collide with the glass with a lot more force than a bullet from a handgun, so a thicker piece of bullet-resistant glass would be needed to stop a rifle bullet as opposed to a handgun bullet.