Since "kung fu" is not a martial art and simply means the mastery of a discipline, I'm wondering firstly which martial art is known in the public eye as kung fu and secondly which martial art is actually taught under this name.
The Chinese term "Kung fu" (gong fu) usually refers to martial arts, though it also more generally means skill attained through hard work that can be applied to any skilled trade. When referring to martial arts, the term does not refer to a single martial art but can refer to many different martial arts styles.
Chinese martial arts tend to be less organized than their Korean (tae kwon do), Japanese (karate, judo, aikido), and Western (boxing, fencing, wrestling) counterparts. Organization means there is a governing body that makes decisions. Without organization, variation is very high.
The term kung fu is applied to styles including, but not limited to:
- shuai jiao
- wing chun
- hung gar
- white crane
- xing yi
Many of these styles themselves can be subdivided into distinct substyles.
I personally would consider the application of "kung fu" to styles such as karate to be correct when considering martial skill earned through hard work, as opposed to classification of martial arts. Karate is not normally referred to as kung fu because it is named from Okinawan or Japanese origins, which use different terminology.
Technically the Chinese term "Kungfu" (spelled and pronounced numerous ways) means "skill obtained through hard work and diligent study" as stated in the previous post.
However, if you use this term in China it will be always be assumed that you are speaking of "Chinese Martial Arts" and nothing else.
There are other terms in the language that are used too. These often have specific connotations in mind: "Wushu" - for the performance side of martial arts, "Sanshou" - free fighting (no weapons used), "Kuo Shu" - a general term for combative martial arts (not necessarily respectable or earned through hard work and study), etc.
Though you could technically make a case for using the term "Kungfu" when referring to a martial art of any origin, that might appear dishonest frankly. Kind of like calling a person an "Engineer" when what they really do is collect garbage from the streets ("Sanitary Engineer").