Kevlar is a material used in making bullet-proof vests, but it's a slang term for body armour (eg. "I'm putting some kevlar on before I get shot"). Flak jackets are designed to stop bits of shrapnel, flying debris etc. and were developed in WWII for bomber crews, so that they'd have a better chance of surviving if they had to go through an enemy barrage. Unfortunately, most WWII bombers were too small for their crews to wear both flak jackets and parachutes, so it was a choice between not getting torn open by shrapnel but plunging to your death, or bailing out and getting opened up by flak :-\