Wiki has a pretty good outline of the JKD principles.
1. Be Like Water
(Water can flow/crash easily; things most martial artists strive to be able to do well)
2.Economy of motion
(The fewer movements between your attack/parry/gaurd/etc. and it's completeion while retaining power/speed/general effectiveness, the better.)
3. Learn the four ranges of combat.
(Do any research into Gracie Jujutsu, and you will read about the stories of how the knocked the stuffin' outta many a stand-up Martial Arts master because said master was only good at one/two ranges. Not to start a flame war over the suprememacy of this or that, but I would say the Gracie family was trying to illustrate a severe deficiency in martial culture: mainly, tunneled-vision specialization. That same idea of not being only focused on X or Y range is presented in this Principle)
4) Five Ways of Attack
-Simple Angle Attack( A single strike to an opponent)
-Hand/Head/[insert extremity here] immobalization attack(I think you got this one...)
-Progressive Indirect Attack (Attacking a different part of the opponent to create a weakness. Example: The Classic High/Low one-two)
-Attack By Combinations (Exactly that: multiple strike sin rapid succesion to thwart an opponent)
-Attack By Drawing (Creating an opening in the opponent's gaurd to generate an opportunity for a counter-attack)
5) Three part of JKD
-Efficiency (An attack that reaches it's mark)
-Directness (Doing what feels natural in a learned way; to quote Wiki)
-Simplicity (Exactly that. You move extremetiy, you perform move, you finish. Acrobatics and flash in between is generally avoided.)
6. Centerline (Bruce borrowed this from Wing Chun. Basically like controlling the center of the baord in Chess; preserve your own center while constantly forcing the other guy to have to keep adjusting his. Kinda like throwing him off balance?)
Hope that helps.
Really though, I'd say JKD is personal liberation from partialized conditioning. In that, you no longer view things according to how you have been trained/taught, but according to what you see transpiring in front of you.
"Success is a process, not a destination. Have faith in your ability."~Bruce Lee