The glass transition is the transition of amorphous solids from a hard and brittle state into a molten or rubber-like state. An amorphous solid that goes through a glass transition is called a glass. Supercooling a viscous liquid into the glass state is called vitrification.
The glass transition temperature Tg is always lower than the melting temperature, Tm, of the crystalline state of the material, if one exists. Above a glass transition temperature, a plastic is rubbery. Below the glass transition temperature, a plastic is solid. Thermosets, or thermo-setting plastics, burn up before they reach their melt temperature, but most thermosets also undergo glass transition.
Media related to Glass-liquid transitions at Wikimedia Commons