The term bullet proof vest is actually a misnomer. There aren’t any vests that are completely bullet proof; but there are vests and body armor which can be considered bullet resistant. These vests, made out of artificial materials like Kevlar which was discovered by DuPont chemicals, are made very specifically so that the bullets not only don’t penetrate, but the force of the bullet is dispersed over the entirety of the material and away from the wearer.
When a bullet strikes a target a huge amount of force is striking in a very small area at the head of the bullet. That force is what causes damage, and the bullet is pushed through material by that force. What stops a bullet from penetrating is when something can absorb and dissipate that force. Now picture a magnified view of cloth. Kevlar is woven like any other cloth, but the material is tough and the weave is so tight that it creates a very difficult to break web. Now picture roughly 40-70 layers of that web. When the bullet strikes it is being slowed by one layer after another and the force of the shot is dissipating over the entirety of the vest, which is stopping it from penetrating and causing the bullet nose to mushroom out and deform upon impact. So what’s really happening when someone’s vest takes a hit is that the bullet’s force is being absorbed by layer after layer of chemically woven spiderweb that’s just too tough to snap, but it can take the shock and spread it over a bigger surface.
However, just because a bullet doesn’t penetrate the Kevlar weave doesn’t mean it can’t hurt you. The blunt force of a bullet, even spread out over a wider area, can still cause heavy bruising and internal organ damage that might be just as deadly as gunshot wounds. Smaller caliber bullets like those used in common handguns like the nine millimeter or thirty eight caliber handguns can be absorbed by simple Kevlar vests. Larger caliber bullets such as 30-06 rounds though, or specially crafted rounds like black talons, are much more likely to penetrate vests made of Kevlar and worn under shirts for emergency protection. Body armor that’s meant to absorb and stop larger rounds is made of Kevlar combined with plates of harder material like specially formed plastic or ceramic that will absorb more shock and help prevent penetration along with the fabric. This sort of body armor isn’t as flexible due to the addition of rigid front and back plates, but it offers protection against a much higher caliber of weapon in addition to offering more protection from other injuries like stabbing and slashing weapons and explosive impacts.
“How Bullet Proof Vests Work,” by Anonymous at Tech Faq
“How Does It Work?” by Anonymous at Body Armor News