Polystyrene (PS) is a versatile plastic used to make a wide variety of consumer products. Polystyrene also is made into a foam material, called expanded polystyrene (EPS) or extruded polystyrene (XPS), which is valued for its insulating and cushioning properties. Polystyrene is a hard, brilliantly transparent, stiff resin. It is produced by the polymerization of styrene.