There are two major parts of any helmet in order for it to protect the rider’s head. There’s the outer shell that is specially made to resist impacts, penetration, or abrasion, and the inner shell that needs to absorb the shock of the impact. Effective outer shells are made of thermoplastic material (usually polycarbonate), fiberglass, or carbon fiber.
The thermoplastic alternative is the most common one, fiberglass is usually quite heavy compared to other ones, while carbon fiber is the most expensive but also the lightest option. The inner shell usually includes polystyrene foam with superior properties, but essentially the same product that keeps your coffee warm.
Keep in mind that helmets should be replaced after a while, so make sure that you ask the manufacturer for this information as well. Plus, you should try to avoid dropping the helmet, as this can start making it less effective in absorbing shocks.