Louis and Siggy from Jarfum.com give a great breakdown of the difference between the different safety standards you find on helmets these days.
Louis and Siggy then give a very good breakdown of the differences of the safety standards and what each of them cover.
One last point they make is that no matter how a helmet is rated, if it doesn’t fit correctly, it may not provide the adequate level of protection. So a good fitting helmet is just as important as it safety rating.
For more information, see my article on helmet safety standards.