It seems that every year a new system of windshield repair is introduced and touted as the newest and greatest yet. There are three different basic types of windshield repair systems. The first is the kind you can find at your local AutoZone-type parts store and will fix most windshield chips. The next kind of kit is more advanced, will repair chips and cracks, and is designed for the auto repair shop that wants to make extra money by repairing windshields. The next type of kit is meant for the windshield repair professional, who specializes in only windshield repair. All of these kits work by injecting a polymer into the damaged section of the window that when dry, completely erases the damage.
For more info, see Auto Windshield Repair Guide: What to Know