Jun Fan Gung Fu is the martial art developed by Bruce Lee whilst Jeet Kune Do is a vehicle of discovery which utilizes Jun Fan Gung Fu to experience the concepts and philosophies of the Martial Arts.
The reality is that Bruce Lee was constantly developing, changing and personally evolving and therefore so was his art and philosophy undergoing the same rate of change over the years.Bruce Lee said "Jeet Kune Do is a name - don't fuss over it" and, like his student Dan Inosanto, believed that the man is far more important than any established system or style.
Jun Fan Gung Fu takes the name of its founder - Lee "Bruce" Jun Fan. It is based on the core principles of Wing Chun learnt in Hong Kong under the tutelage of Sifu Yip Man in the 1950's. There is also some discussion that Bruce Lee also learnt much from class seniors Wong Shun Leung and William Cheung - in all probability in the same manner that any student would have learnt from their class seniors. Although the basis of Jun Fan Gung Fu is certainly Wing Chun - it is well known that Bruce Lee also practiced and competed in boxing as a schoolboy and experienced Tai Chi through his father and his father's friends.