Kung Fu is any technique or martial art with Chinese origin, thus, Chinese martial arts are commonly divided into two categories: Internal and External or hard and soft styles. The earliest distinction between the two was recorded in the "Inscription For Wang Zhen Nan", written in the early Qing Dynasty, where the Shaolin school of martial arts was called the External system because of its techniques focused on attacking the opponent. Thus, the Wudang school of martial arts founded by Zhang San Feng was called the Internal School because they used to neutralize opponent force in a tranquil manner.
Later, schools which originated from Shaolin school were generally classified as External martial arts and those who were originated from Wudang school were generally classified as Internal martial arts.
Also, those techniques that focused on physical strength were classified under the External system and those that focused on internal force were classified under the Internal System. This doesn't mean that within schools considered in general as external there are no internal elements or vice versa.
Most of known Chinese martial arts in occidental countries are considered external martial arts; in the other hand, internal martial arts are less known, some examples are Pa Kua Chang, King Yi Quan, Liu He Ba Fa and Tai Chi Chuan or Taijijuan as it is spelt in Romanized Chinese. The last one is perhaps the most known and it means "the grand ultimate", and figuratively "the cosmos" Because Tai Chi is more commonly used as a way to get relaxed and keep good health, some people don't know that it is actually a martial art at all. The amazing thing about Tai Chi Chuan is that to defend yourself against almost any form of physical aggression, you need to know only a few fighting patterns.
In an internal martial art like Tai Chi, the power is generated in the same way one would push some heavy object, for example a bookshelf, you push with your legs using your arms, this means, you are transferring the legs power to the object you want to push, using the power of the whole body, this explains why Internal martial arts power is commonly named "whole body power". The difference between pushing the loaded object and striking it, it is the speed.
Tai Chi have been characterized by meditation and spirituality, in this way, practitioners can develop and learn to control the "chi" or vital energy, which flows all over the body. Besides Tai Chi practitioners learn to use opponent's power against itself, redirecting the energy.
Even the division of martial arts into External or Internal, an important concept in Tai Chi Chuan is Ying-yang, in many ways, Tai Chi is all about it, if there is only yin and no yang, or vice versa, then that is not Tai Chi Chuan, thus a good Tai Chi practitioner has to develop both stillness and physical force to get all advantages of this Internal martial art. This same concept should be applied to all martial arts, this is why without dark, there would be nothing that could be called light.
