it's a system that this Jewish P.E. teacher made in 1930's Europe...he emigrated to what was then Palestine before WWII I believe...
he began teaching it to Jewish land owners in Palestine and eventually after the creation of the Israeli Defense Forces began teaching it in the army as basic hand to hand training.
it proved to be wildly effective and successful. it's a MMA and hybrid system that emphasizes getting oneself out of danger...so the main focus is ending the fight as quickly as possibly whether it be hitting, kicking, choking, eye gauging, stabbing, maiming, killing or running from your opponent.
another cool thing about krav maga is the 360 defense-- a technique for blocking and delivering counter strikes at the same time.
they borrowed a lot of punching from wing chun (short, vertical punching) boxing/kickboxing from muay thai and savate, theres also SO much judo and jiu-jitsu.
i think the some of the most groundbreaking (no pun intended) techniques are the grappling techniques and especially the weapon disarms. there has been a lot of evolution in jiu-jitsu since it left Japan in the 20's and 30's. krav maga has continued this evolution and because of its military history has combined and/or created tons of grappling techniques for weapon disarming. a lot of the grappling is an extension of what one might learn in aikido or jiu-jitsu but unique to krav maga. these have proven so effective krav maga is used to train law enforcement world wide, almost uniformly...
it's basically combat Sambo on PCP.