Bulletproof glass is a colloquial term for glass that is particularly resistant to being penetrated when struck by bullets. Since manufacturing glass of usable thicknesses capable of fully stopping most bullets cannot currently be done, the industry generally refers to it as bullet-resistant glass instead. Bullet-resistant glass is usually constructed using a strong but transparent material such as polycarbonate thermoplastic or by using layers of laminated glass. The desired result is a material with an appearance and light-transmitting behavior of standard glass but offers varying degrees of protection from small arms fire. The polycarbonate layer, usually consisting of products such as Makroclear, Cyrolon, Lexan and Tuffak, is often sandwiched between layers of regular glass. The use of plastic in the laminate provides impact-resistance, such as physical assault with a hammer, an axe, etc. The plastic provides little in the way of bullet-resistance. The glass, which is much harder than plastic, flattens the bullet and thereby prevents penetration. This type of bullet-resistant glass is usually 70–75 mm (2.8–3.0 in) thick. Bullet-resistant glass constructed of laminated glass layers is built from glass sheets bonded together with polyvinyl butyral, polyurethane or ethylene-vinyl acetate. This type of bullet-resistant glass has been in regular use on combat vehicles since World War II; it is typically about 100–120 mm (3.9–4.7 in) thick and is usually extremely heavy. Cost is a factor in this. Current types of bullet proof glass cost about three dollars per square inch, while ALON costs about five times more. Source: Google search,Wikipedia Also see: http://science.howstuffworks.com/question476.htm

jacks answer is great & the cost of it is saving someones life,(priceless) :)