Polystyrene is a transparent, colorless thermoplastic. It's commonly used to make foam board, concrete block and loose-fill insulation.
Expanded polystyrene (EPS) is a specific type of polystyrene made of small plastic beads fused together. It's typically used in concrete blocks. In contrast, molded expanded polystyrene (MEPS) insulation is available as small foam beads. MEPS is commonly used for foam board insulation. Although it can easily be poured into hollow wall cavities, MEPS is notoriously challenging to control -- a small hole in the wall or a gust of wind will send the foam beads scattering all over. Foam board polystyrene's thermal resistance depends on its density and usually ranges from R-3.8 to R-5.0.