Tempered glass is made by subjecting glass to thermal treatment or by treating it with 
hydrofluoric acid. This kind of treatment increases the compressive tensile on the surface of the glass making the surface toughened. This makes tempered glass resistant to heat and pressure applied on the surface. Unlike normal glass which breaks and separates the pieces apart, when tempered glass breaks, it shatters into very smaller pieces, which may still be intact or split depending on the surface coating.

These screen covers can withstand a lot of heat that may emanate from keeping the phone in a pocket that gets heated by the body, without undergoing thermal expansion. This is a property that makes is better than plastics and other forms of screen protectors, which easily undergo thermal expansion and damage. Because the glass does not heat up, the adhesive keeps to the surface even when heated. Plastics will begin to peel off the screen when heated a little.

Applying a protector that is tempered glass to your phone or tablet surface is fairly easier than plastics and others. This is because, the glass is stiff and does not stretch during the application. Most of the screen protectors are already cut to the size and shape of the device they are made for and so one action of sticking is all you need. Plastics on the other hand, because of their simple expansion property can stretch during application. Some of them may require re-shaping afterwards in order to fit the device properly.

Smudging during screen protector application is one problem which most phone and tablet owners face. With plastics, because of their thin nature, it is easier to get crumpled and pick up air bubbles. This makes them very clumsy and messy at times. Tempered glass screen cover application on the other hand does not present this problem of smudging. Applying plastics will require a professional and experienced person to do perfectly, whiles the tempered glass can easily be done by the owner of the device without any difficulty.

They also provide the smooth touch feel of the screen that you feel with the original screen of the device. With plastics however, you can feel the difference because the swipe and touch is not that smooth.

The only problem with the tempered glass cover is that, it cannot be reshaped or resized easily like the plastic ones. Tempered glass is usually shaped before it is toughened. It is not possible to even cut it after that. This means that, you must buy a premium tempered glass screen protector made specifically for your device. With plastics on the other hand, you can cut and reshape one that is not made for your device to fit its surface without any problems.

source : Tempered Glass Community