Auto Glass is not "Just Glass"
Safety glass is used in all automobile glass. It is manufactured to reduce the likelihood of injury if it breaks. There are two different types: laminated glass and tempered glass.
Windshields are made from a lamination process. A windshield consists of two pieces of glass, bonded together by a vinyl inter-layer. In the event of a crash, the vinyl layer will cushion your head during impact. If a windshield breaks during impact, the broken pieces will generally adhere to the plastic lining.
Side and rear windows consist of tempered glass which is produced by heating the glass to more than 1,100 F, then rapidly cooling it. This results in the outside surfaces of the glass becoming harder than the center of the glass and, therefore, making it stronger than regular glass of the same thickness. If broken, tempered glass will break up into very small pieces.