Polystyrene is commonly produced in three forms: extruded polystyrene, expanded polystyrene foam, and extruded polystyrene foam, each with a variety of applications.

Extruded polystyrene
Extruded polystyrene (PS) is economical, and is used for producing plastic model assembly kits, plastic cutlery, CD “jewel” cases, smoke detector housings, license plate frames, and many other objects where a fairly rigid, economical plastic is desired. Production methods include stamping and injection molding.

Foams
Polystyrene foams are good thermal insulators, and are therefore often used as building insulation materials, such as in structural insulated panel building systems.
They are also used for non-weight-bearing architectural structures (such as ornamental pillars).

Expanded polystyrene foam
Expanded polystyrene foam (EPS) is usually white and made of expanded polystyrene beads. Familiar uses include packing “peanuts” and molded packing material for cushioning fragile items inside boxes.

Extruded polystyrene foam
Extruded polystyrene foam (XPS) has air inclusions which gives it moderate flexibility, low density, and low thermal conductivity.