So, what is
Glasses can be manufactured in different ways and from different materials. They can be organic, inorganic, or even metallic. As such, a glass cannot be defined by its chemical nature. All glasses have some characteristics in common though: they have an amorphous structure and, over a specific temperature range, a certain kind of behaviour that is known as glass transition. Thus, a glass is defined as an 'amorphous solid that shows a glass transition'.
Most kinds of glass that are in daily use (the kinds that you drink your beer out of, the kind that you look through when you look outside and the kind that is used in laboratory glassware), are oxide glasses. They consist of a network of (usually metallic) ions, connected by oxide ions. The prototypical oxide glass is amorphous SiO2 or fused silica.In this material the basic unit consists of a silicon ion surrounded by four oxygen ions in the shape of a tetrahedron. The oxygen ions form the bridges between the units; each 'bridging' oxygen is shared by two tetrahedral units so that the overall formule turns out to be SiO2, not SiO4 as you might suspect from the basic unit.
The presence of the 'bridging' oxygen ions makes a three-dimensional network structure possible. This kind of structure is characteristic of oxide glasses, although the shape of the basic unit may vary.
Pure fused silica has a very high melting temperature and is therefore hard to process. To remedy this, its structure is modified by adding Na2O. Each Na2O molecule reacts with the silica network by breaking up the network at the site of a bridging oxygen ion. The result is then one complete and one incomplete silica tetrahedron. The oxygen ion from the Na2O molecule fills up the space in the incomplete tetrahedron, resulting in two so-called non-bridging oxygen ions. The local excess space charge this causes is neutralized by the Na+ ion. The addition of a modifier (there are others than Na2O, although Na2O is widely used), causes the glass network to become 'looser', making processing of the glass easier. Besides that, the presence of the Na+ or similar ions makes diffusion of ions through the glass possible.