All helmets are created equal. Or are they?
There are four main types of helmets. They all have to meet the same CPSC safety standard in the US, so in a way, yes- all helmets are created equal. However, different types of helmets provide different functionalities.
The main rule about bike helmets? They can only withstand one (1) crash or ~4 accident-free years, so you should replace them regularly.
The most common type of helmet out there. EPS foam provides impact absorption while a polycarbonate outer layer protects the foam from abrasion. Weight and ventilation are the biggest factors upon which price is dictated upon: the lighter and more ventilated, the higher the cost.
These bad boys are meant to withstand high strength impacts and are designed to protect the head from most angles of impact. They are made with a hard TPU shell and EPS liner, meaning they too should be replaced after every crash.
Along the lines of skate BMX helmets are full-face helmets. They provide maximum coverage and are only used for gravity MTB racing and BMX. They too should be repalced after every crash.
A combination between Road and BMX helmets, MTB helmets are a relative newcomer. They provide serious coverage and include large ventilation vents to provide well-rounded performance.
Aero helmets are the most recent addition to the helmet lineup. They are made to be as aerodynamically efficient as possible as seen by their teardrop shapes and slick air channeling. Aero helmets with protrusions are only allowed in Time Trials.
How To Wear Your Helmet