What is Kung Fu?

Chung Kuo Wu I (Chinese Martial Art)

Kung Fu is the whole of Chinese Traditional Martial Arts. The term Kung Fu literally means an exercise which requires a constant effort, energy, devotion and executed with ability.
Chinese often use other terms to indicate Martial Arts:
 

Wu I Martial Art Wu Shu Martial Art Kuo Shu National Art Chung Kuo Ch'üan Chinese Boxing Ch'üan Shu Fist Art Ch'üan Fa Fist Method

Kung Fu is the most used term in Occident and Hong Kong.  In Taiwan Kuo Shu in more used.  In Popular China, the official term is Wu Shu, but since in this country martial arts have recently undergone deep changes, it would be better to distinguish between "modern" Wu Shu and "traditional" Wu Shu.

 
Kung Fu is made up of an incredible number of different styles, that have been divided into two big families, called Wai Chia and Nei Chia.Wai Chia means "External System" and comprises the "external styles", which give prominence to outside features (strength and speed). The most important external style is Northern China Classical Shaolin, from which all other Kung Fu styles derive. External styles have been divided into "Northern styles" and "Southern styles" too. The former are usually characterized by wide positions and tecniques, and by numerous kicks and projections. The latter use smaller positions, shorter techniques and less kicks and projections.
The "Internal System" - Nei Chia - comprises the "internal styles", so colled for the importance given to "inner" features, such as internal energy and mental energy. The most known internal style is Tai Chi Ch'üan.

Distinction between Wai Chia and Nai Chia, arisen for historical reasons and quite useful, is nevertheless conceptually wrong. In fact in external styles too the use of internal energy is fundamental. Furthermore the practitioners of internal styles, in martial applications, do not avoid techniques with explosive features of energy and speed. Master Chang used to repeat that in external styles you begin with exterior features to arrive to internal ones, while in internal styles you begin with internal features to arrive to external ones. He also said (and showed many times) that the ones who have reached a very high skill do not make difference between "external" and "internal".