Flak Jacket
The flak jacket is a vest worn over the torso which is designed to withstand shrapnel from a bomb or other low velocity projectiles. Most importantly, it is not a bulletproof vest.
Materials
The main component of all flak jackets is layers of ballistic nylon, a material developed by DuPont. The man-made fiber is weaved in a certain way to create a higher density. It is then made into clothing in numerous layers so it can withstand certain impacts. Through the years metal plates were also put inside the lining of the jacket to give added protection.
Use
Flak jackets are largely discarded now in favor of bulletproof body armor. The idea was first used during the First World War by pilots to protect against high-explosive shells. They were in much wider use by the Second World War and were widely used by the infantry in Korea and Vietnam. In Vietnam, the metal plates were replaced with a combination of ballistic nylon and fiberglass.
Bulletproof Vest
Today flak jackets are used as a general term for body armor, but modern bulletproof vests are very different from flak jackets. A bulletproof vest is made from Kevlar which can withstand a much more high velocity impact than ballistic nylon and fiberglass.