Polystyrene (PS) is a synthetic aromatic polymer made from the monomer styrene. Basically it is a hard quite brittle plastic, typically used in CD cases, disposable razors, plastic, cutlery etc
Expanded Polystyrene (EPS) is this same brittle plastic that is foamed using a gas, generally Butane or Pentane. EPS is typically used in insulation for cold rooms, packing material, disposable plates and cups etc.