Expanded Foam Helmets: These helmets are largely constructed of expanded polystyrene foam. They are very light and comfortable to wear and it is probable that their invention has greatly increased the popularity of wearing a helmet whilst climbing. On the negative side , a sharp blow from a stone may crack the helmet so badly it can no longer be worn - bad news if you are half way up an alp.
"Plastic" Helmets: Helmets with a tough outer shell of polycarbonate, polythene or other plastic. Most have some internal polystyrene foam padding. The advantage of these helmets is that they can still be used if say, in the alps for instance, they receive a blow from a stone and it is not practicable to replace them immediately. Their disadvantage is that they are generally heavier than expanded foam helmets.