Welcome to Body Armor Protection
Even know body armor or 'personal armor' has been around in one form or another for several hundred years, body armor is becoming more and more powerful to the point where it allows us to protect our body properly without having to over encumber it.
Yes, body armor is also getting thinner thanks to new manufacturing methods that allow materials such as Kevlar for example to be thinned without losing their quality and capabilities which is amazing to say the least.
One of the first recorded descriptions of soft armor use was found in medieval Japan, with the armor having been manufactured from silk. A silk vest once saved the life of Alfonso XIII of Spain from an attacker's bullet in 1901.
Silk is an example of a 'soft' armor. 'Hard' armor is a plate of metal or ceramic. Of the two, soft armour is more comfortable and advances in fibers bode well for further advancement of this type of protection.
Kevlar was introduced in the mid 1970's and is an example of 'high-tech' fibers replacing older materials. New woven fiber body armor materials have been introduced since Kevlar and are better but much more expensive. Surprisingly, fiber based vests' performance is degraded when wet. Ordinary water can foil a high tech bullet proof vest.
The first fiberglass based laminate was used by the US troops in 1945. It was useful for stopping general hazards/shrapnel, but not bullets.
It is often possible to beef up protection of a soft armour by adding a metal or ceramic plate. It is a hard plate that is needed to stop the really stout projectile.
If one is interested in show only, a suit of gnarly looking body armour can be made from PVC plastic pipe. This looks good, but is not much protection against a common bullet.
A 'flak jacket' is intended to stop shrapnel or anti-aircraft flak, but not bullets. Most body armour is intended to stop bullets and/or knives. The notion that the suit is specialized against intended threats and not others is a common one.
A 'bullet proof vest' may stop a bullet, but not the energy of the stopped projectile. The resulting force of the stopped bullet/projectile causes blunt force trauma. It's similar to having a hammer thrown at you, very hard. Backface signature is the amount of blunt force the vest transmits after it stops a projectile.
'Bullet resistant' is preferred, 'bullet proof' implies a vest will stop anything. Because of the wide variety of projectiles that might be encountered, this can be misleading.
The degree of protection depends mainly on the hardness of the projectile - lead is soft and will usually be stopped, tungsten carbide is hard and usually will not be stopped.
Bomb disposal personnel usually wear a heavy full torso, head, neck, arms and hand protectors. Bomb suits are not necessarily good for being bullet proof (again, it depends on the threat being protected against).
Recently, both knife/ice pick and bullet resistant vests have become available. These are of special interest to prison guards. This type of vest will protect against most/all hazards encountered in a correctional facility.
The newer US army vests are part of a complete component system that will provide protection for just about every body part. However, more protection equals more weight to carry around. In Iraq, the case is validly made that it is harder to chase after light weight, poorly equipped guerrillas if a US soldier, with up to 100 pounds of protection, ammo, food, water and other equipment is in pursuit.
On the other hand, the US soldier is much better protected.
All in all, a generally good case can be made for the merits of body armour.