It's a self defense system developed and used by the Israeli Security Forces. They teach it to a lot of cops and special forces people. It's also the new trendy thing to do. Apparently most of the stuff in the Bourne movies is Krav Maga, also the J-Lo movie "Enough" has Krav Maga in it.
It's not really a traditional "martial art." There aren't any competitions or championships, and it doesn't lend itself to "sport" really well. My buddy is a former kick boxer and was with the 7th Special Forces before law school and has had some basic stuff. He really likes it and when we found out one of the law professors is a certified instructor (also former military) he was all stoked to go.
http://www.kravmaga.com/