Jeet Kune Do (JKD) is a non-classical Kung Fu system created by Bruce Lee.  His idea was to take the best of Wing Chun Gung Fu, American Boxing and French Fencing and bring them together as the ultimate combat art.  Techniques as taught by Lee, combat modified Gung Fu with grappling. The first original phase of JKD was Wing Chun. The second phase was combat modified Gung Fu. The third phase was grappling. This is the original JKD.
Bruce Lee not only took the best from other arts to incorporate into JKD but was innovative in the way they combine to form a complete art rather than simply calling his training a mixed martial art. 
The number one rule of Jeet Kune Do is to use whatever works.

There are two schools of thought in JKD practice today ... Jun Fan/JKD practitioners, and the JKD Concepts practitioners. Jun Fan/JKD practitioners concentrate on Bruce Lee's original teachings, training and fighting methods, while the JKD Concepts practitioners use Bruce Lee's ideas and theories and explore and add techniques from many other martial arts to their training.

Originally, Bruce Lee was a child movie star, and later studied Wing Chun as a student of Yip Man sifu in Hong Kong. Later, he informally studied some other Chinese martial arts, as well as the sports of Western boxing, Western fencing and Greco-Roman wrestling. Bruce Lee and Dan Inosanto studied 26 "official" arts (mostly Chinese kung fu styles) and 4 "unofficial" arts during the process of refining Jeet Kune Do. The term Jeet Kune Do actually comes from an off-hand comment Bruce once made about his art being an "intercepting" art.
But one of Bruce's last statements was not to make too much of the name, because the process is what is important, not some product (indeed, some schools now claim to teach the "art of Jeet Kune Do", but that is not believed by other JKD teachers to be in Bruce's original concept of the "art" being a process).

Physical blocking of an incoming blow is only used as a last resort by the JKD practitioner ... instead he uses the four corner parry which redirects the incoming force. The best defense in JKD is to attack!! The next preferred method of defense is the simultaneous attack and defense whereby you parry the opponent's attack while delivering (at the same time) an attack of your own to the open line. An even more effective form of defense is to fire a fast powerful attack of your own into the same line as the incoming attack, thereby deflecting the oncoming attack and landing successfully on your target - This form of interception is called the stop-hit; when using the foot for interception it is called a stop-kick. When you have honed your interception skills, damage is done immediately to the attacker, both mentally and physically.

Once in close range, head butts, elbows and knees are used. This is generally where close quarter grappling occurs (chokes, strangles etc.) and is a very deadly range due to the serious nature of the natural body weapons that can be employed.

Kicks, punches, trapping and grappling movements are used at medium range.  As a general rule, by the time your opponent is moving into the medium range you should have already intercepted him and countered his offense with an attack of your own.

The most important factor in JKD training is sensitivity training. Every offensive and defensive movement will have a certain type of energy and energy flow. Sensitivity drills that are used in JKD are referred to as Chi Sao or "Sticking hands" and use of this drill will enable the student to "sense" the opponent's energy quickly and subsequently trap and counter him immediately. Chi Gerk or "Sticking legs" develops the sensitivity in the legs for sweeps, deflections and counter kicks practitioners.

Jeet Kune Do is Bruce Lee's personal interpretation of combat, originally called Jun Fan Gung Fu. Its practitioners claim that it is not a system of martial art, but an interpretation of combat from a personal perspective. Bruce Lee emphasized that Jeet Kune Do was to be understood as a process, not a product (another way of stating the claim that Lee's interpretation was different from others' interpretation of "martial art").