Jeet Kune Do is unique among martial arts, as it is the first to advocate total freedom for its practitioners and the absence of "form."
Jeet Kune Do is a philosophy developed by martial arts master Bruce Lee. During his kung fu training, Lee became disenchanted with the rigid styles and forms taught by martial arts schools. While the fundamentals of each style were important building blocks, to Lee they became barriers to true mastery. He began combining elements of Wing Chun, Western Boxing, Fencing and thus Jeet Kune Do was born (since 1967).
Jeet Kune Do is not a particular style of kung fu or other martial arts, nor is it a series of techniques; it is a philosophy on living life at the pinnacle of development. Jeet Kune Do allows a martial artist to create his own expression. There aren't particular rules that make up the style -- it is a truly free art, encompassing everything, yet possessed by nothing. It rejects the rigid forms and katas used by other martial arts, and instead stresses spontaneity and intense training of the body for peak performance.
Lee created a style of kung fu called Jun Fan (which was his birth name) to work in conjunction with Jeet Kune Do. This style can be thought of as a more practical application of the philosophies of Jeet Kune Do. It is a pragmatic and free-form approach that teaches students to be competent in all arenas of fighting.