Most helmets feature an outer ABS plastic shell lined with EPS foam; construction is either in-mold or injection-molded. These helmets are designed to absorb the impact from a single catastrophic event, after which it should be discarded. A few manufacturers (like Bern) offer models with softer foam that absorbs multiple low-intensity hits, but these aren't rated to the same safety standard.
Your helmet should fit snugly. If it feels loose when you shake your head, it's too big. Most helmets use Boa dials, sliders, or removable padding so you can make adjustments to ensure a good fit. Always measure your head and use the size chart first, though.
Most helmets feature venting to keep your head cool and comfortable. Venting systems vary, but usually consist of outlets that allow hot air to escape and inlets that allow cooler air to circulate. Often venting can be controlled with sliders or with drop-in plugs.
If you like listening to music when you ride, or want hands-free convenience for phone calls, look for ear flaps or attachments that add audio capabilities to your helmet. Some helmets come with audio built in, while others are simply audio-compatible, with speaker units available separately.