In order to form glass, two things are important and necessary. 1) Glass Former and 2) Glass Modifier.
Chemicals like B2O3, TeO2, P2O5, SiO2 are used as Glass former whereas Alkali-Alkaline Earth Oxide/Fluorides/Chlorides, MnO2, Bi2O3, ZrO2, Al2O3 etc are used as glass modifier.
Glass Former helps to form glass on the other hand to give thermal, chemical, mechanical stability, modifiers are used. For example Al2O3 is well known modifier which mainly helps the glass to have good mechanical strength.