I'm currently reading The Art of Shaolin Kung Fu, which I am enjoying throughly. It gives a great background the the many forms of Kung Fu, or more respectively known as, quanfa, (q=ch, as in chanfa, art of the fist). The book reminds me of a philosophy I once learned before.
Many also associate Kung Fu with games like Mortal Kombat or Jackie Chan movies. Though Jackie Chan is a master of Northern styles, there are over 1,000 styles of the art originating in China, differing from family traditions to regional traditions. All of the forms of Quanfa have their origins from the Shaolin Monastery.
From your thoughts, your actions, to your relationships, Kung Fu is not just about punches, kicks, and back-flips. It's a way of life, no different from the Japanese Bushido (Way of the Samurai). Its philosophies derive from Buddhist and Taoist scriptures, and are in fact, very intertwined.
(And please don't assume all Buddhists or Taoists are martial artists and vise versa)
But why would a Buddhist or Taoist want to harm another through martial arts? We don't want to. Does a thorn from a rose not protect itself from those who want its blossom? One of the laws of Shaolin monks is to NEVER kill. We as humans can defend ourselves, but should never kill. Martial arts are to be used to cease or subdue conflict, to bring peace. The other reason of practicing the art is one of self preservation and self discipline. Kung Fu is beneficial to health, longevity, cultivating Chi, and moral behavior.
Technically, you don't even have to be a martial artist to practice literal "Kung Fu." Kung Fu can be in the writer, the painter, or the car mechanic. It's ultimately about perfecting what you do and how you do it. The martial artist, then, technically practices Quanfa, while utilizing the philosophy of Kung Fu.
So now you know the true meaning of Kung Fu/Gung Fu. Below is a great interview with legendary Bruce Lee discussing some of these points: