Windshields are most often made of safety glass that will not easily crumble or fragment into loose pieces on impact – to protect the vehicle's occupants from further damage. Modern windshields are frequently made with two layers of curved glass on either side of a plastic laminate layer. The entire piece is then bound to the car using polyurethane.
When windshields are repaired, they are filled (via drilling and injection) with a resin that mimics glass. The resin has an index of refraction that nearly matches that of glass; therefore, when light shines through the glass, the repaired area almost disappears. Most repairs do leave a small 'scar', however, and major cracks or damage that interfere with the driver's sight should be discussed with a professional. It may be necessary to replace the windshield in these cases.
Check out our frequently asked windshield questions.