When you're at the beach and you pick up a handful of sand, a large percentage of the tiny bits of stone in your hand are quartz, a crystal form of silicon dioxide.
Quartz (or Silica) is so abundant because it's constituent elements (Oxygen and Silicon) are among the most common elements in the earth's crust.
The elements react together to form silicon dioxide molecules (SiO2). A quartz crystal is an arrangement of these SiO2 molecules, in the same way that an ice crystal is an arrangement of H2O molecules.
Heating up quartz gives the SiO2 molecules energy and they vibrate. If you keep heating them, their vibrations will eventually reach a critical value (melting point) and they become liquid SiO2. With the addition of other chemicals (depending on the type of glass you're making), you get molten glass.
Glass is a special kind of solid known as an amorphous solid. This is a state of matter in which the atoms and molecules are locked into place, but instead of forming neat, orderly crystals, they arrange themselves randomly. As a result, glasses are stiff like solids, yet have an arrangement of molecules like liquids.
Amorphous solids form when a solid substance is melted at high temperatures and then cooled rapidly -- a process known as quenching.
In many ways, glasses are like ceramics and have all of their properties: durability, strength and brittleness, high electrical and thermal resistance, and lack of chemical reactivity.