Jeet Kune Do is a form of martial arts created by one of the most awesome people to ever live, Bruce Lee.  It loosely translates to "The Way of the Intercepting Fist".  Awesome!  So what is it exactly?  Well...kind of hard to find a good answer.  The following quote is from the World Jeet Kune Do website:

The original art itself is a modification of Lee's first martial art style of Wing Chun Kung Fu. So many modifications in fact that it is very hard to see some of the similarities of the two systems. The blocks and hand manoeuvres such as grabbing, sticking, and energy techniques have their roots in Wing Chun but the finished product is pure JKD. JKD has had such an influence in the martial art word the even the core art of Wing Chun has adopted JKD sparring techniques. The second of the three arts in the core of original JKD is French Fencing. Who can deny the speed and agility in the art of fencing? The footwork is a combining and modifying of fencing, Wing Chun, boxing movements, placements and displacements. And the final art of Western or American Boxing for the Muhammad Ali hand manoeuvres and punches.

Well, that clears it right up.  From the same website comes this quote, which makes more sense:

It was born from Lee's idea to take the best of Wing Chun Kung Fu, American Boxing, French Fencing and Grappling to bring them together as the ultimate combat art, from the ultimate combat artist.

According to the Bruce Lee Foundation 

Jeet Kune Do’s stance, footwork, and major strategic points come from fencing.  A key principle in fencing, the stop-hit, is essentially the JKD namesake—the way of the intercepting fist.  The idea that you can set up your opponent so that you will be able to intercept him in his most vulnerable state—on the attack...JKD’s vertical-fist jab, proper alignment, striking surface, hip rotation, and kinetic chain sequence all come from boxing.