Depending on the level of vest/helmet, you can stop anything from small pistol bullets to large caliber rifle bullets. For example, I believe a kevlar vest rated as a class 1, something that can be very thin, almost an undershirt, will stop 9mm up to a certain distance. As you go up, the fabric gets thicker and heavier, but more capable of stopping bullets with higher amounts of kinetic energy. Note that any bullet, even if it doesn't pierce the vest, will probably at best sting like mad if not incapacitate you from the pain. It's more for preventing a casualty than anything else.
Ablative plating (I think that's the generic term for the ceramic plates that make up much stronger armor) can stop much, much larger rounds by nature of it fracturing and dissipating the kinetic energy of a bullet away from you. Generally the plates have to be replaced after absorbing shock.
I don't know as much about helmets, although I've had a buddy in combat in Iraq that got shot in the head by a 7.62x39, he blacked out for a minute or so but was otherwise fine, so I'm guessing there's similar tiers for helmets as well. Moreover than any of this information on armor is simply knowing how to reduce yourself as a target though, all the armor in the world isn't going to do a bit of good against someone who's 10 feet from you with a gun with your back turned to him. Presenting yourself as a small of target as possible (IE Distance), sight blocking objects and thick, dense material, and knowing the closest unblocked exits to any location will help just as much as a 4 inch plate on your chest, and the two are at their best when combined together.