There are two different levels of coverage: three-quarters and full face. WHichever style you choose you get most protection by making sure that the helmet

Meets US department of transportation(DOT) and state standards(Helmets with a label from the Snell Memorial Foundation gives you an added assurance of quality)

Fits snugly all the way around

Has no obvious defects such as cracks, loose padding or frayed straps.

Whatever helmet you decide on, keep it securded fastened on your head when you ride. Otherwise it may fly off your head before it gets a chance to protect you