With this post I would like to point out a new series of publication called “What is …?” in which, I will give my definition of the proposed term, as always supported by references, and sometimes even, some anecdotes which will illustrate my point of view.
So I start this series with the question : What is Kung Fu 功夫? Question that I have answered dozens and dozens of times to many people who have asked me about my martial practice.
History and definition of Kung Fu 功夫
History first. The term Kung fu, or rather Cong Fou, was first used in the West in the 18th century, by Jesuit missionaries returning from Beijing, to present their work on Chinese culture at the King of France’s court.
In 1779, the Jesuit father Joseph Amiot indicated that the Cong Fou is: “a practice of medicine that can be used for the relief and cure of some diseases. […] The Cong-fou consists of two things : in the posture of the body, and in the manner . The Cong Fou described by Father Amiot made reference to the practice of Chinese health that we know today under the term Qi Gong 氣功, or Dao Yin 導引. Therefore, it is very different from the representation that we currently have.
Concretely, what does Kung Fu 功夫 (in Cantonese) or Gong Fu (in Mandarin) mean ?
Kung 功 means work, mastery, effort.
Fu 夫 means accomplished man, by extension husband, master.
We can translate Kung Fu 功夫 by accomplished work, hard work, man accomplished by his work  or as part of martial arts ; train hard and well . For my part, when I explain the concept to my students, I use this formula ; acquire talent, mastery, skills, through good work and many efforts.
Personally, I like the definition given by the character Bayan Undred Eyes in the Marco Polo series.
“Supreme skill from hard work” Bayan Undred Eyes, Marco Polo series (S01Ep3)
♫ Everybody was Kung Fu fighting ♫ sung by Carl Douglas in 1974. And even today, the general public very often assimilates the term Kung Fu to a dynamic and athletic martial art. Post-war Hong Kong martial arts films, and recently the shows of Shaolin monks around the world, have strongly nurtured the image of the disciplined warrior monk, capable of extraordinary physical and martial prowess.
However, the term Kung Fu does not designate a particular martial art. In China, it is a colloquial expression that can also be used to refer to anything related to work or technique, in any field. When we talk about Kung Fu in China we are not necessarily talking about martial arts. So a taxi driver can have good Kung Fu in driving and a chef can have good Kung Fu in preparing delicious food. Chinese martial arts are made up of techniques, so they are sometimes referred to as Kung Fu in spoken Chinese.  
Bruce Lee may be one of the first to use the term in modern times . In 1960, he named Jun Fan Gung Fu 振 藩 功夫 the method of combat that he taught to his American students in Seattle . Jun Fan being the Chinese name of Bruce Lee and Gung Fu the spelling used to transcribe 功夫. In 1963, Bruce Lee published his first book titled Chinese Gung Fu The Philosophical Art of Self-Defense.
Why did Bruce Lee use the spelling Gung Fu and not Kung Fu ? Because the Taishan people of Guangdong province, were the first to settle in America, especially on the west coast. Their accent strongly influenced the Cantonese spoken in American chinatowns. So it is the sound [g] which is pronounced to say 功夫, hence Gung Fu, whereas in Hong Kong, it is pronounced more with the sound [k], hence Kung Fu. 
“Chinese speakers seldom use the term gongfu, except when speaking English.” Peter Lorge 
In China, Chinese martial arts are referred to only by the name of the style. We talk about Wing Chun, Hung Gar, Choy Li Fut, Tang Lang, Bak Hok, Baji, Tong Bei … Sometimes the term Kyun/Kuen (in Cantonese) or Quan/Chuan (in Mandarin) is added behind the name of the style. Kuen or Quan 拳 literally means fist and by extension boxing. We can then talk about Wing Chun Kuen (Wing Chun boxing), Hung Gar Kuen (Hung family boxing), Shaolin Quan (Shaolin boxing), Taiji Quan, Xing Yi Quan … Only the Westerners use the term Kung Fu after the name of their style. Myself, I designate my art as Wing Tsun Kung Fu, as others can speak of Shaolin Kung Fu, Pak Mei Kung Fu or Hung Gar Kung Fu. But apparently, you will not find Wing Tsun Kung Fu 詠春功夫 in Hong Kong. Here are some examples of Wing Chun 詠春 school name from Hong Kong : Chiu Hok Yin Ving Tsun Martial Art Association 趙學賢 詠春拳 會, Ving Tsun Athletic Association 詠春 體育 會 or International WingTsun Association 國際 詠春 總會.
When Chinese people talk about martial arts, generally speaking, they use the term Wushu 武術 (meaning martial arts). During the period when the Kuomintang dominated the Republic of China, from 1928 to 1949, the term Guoshu 國術 (meaning national arts) was used. When the Communists took power in 1949, they abandoned this term and replaced it with Wushu. 
The different classifications of Chinese martial arts
In the West, the term Kung Fu 功夫, like that of Wu Shu 武術, has become a generic term for Chinese martial arts . There are several classifications to designate them. The three most commonly used classifications are : internal/external, north/south, and traditional/modern.
This classification seems to be the oldest. As early as the 17th century, Huang Zongxi 黃宗羲 and his son, Huang Baijia 黃百家, made mention of the so-called “internal arts” neijia 内家, thus opposing the “external arts” weijia 外家. The Huangs were heirs to the Wudang martial arts created, according to legend, by Zhang Sanfeng 張三丰. Huang Baijia wrote in 1676 : “Shaolin is the peak of refinement for the external arts. Zhang Sanfeng was a Shaolin expert, but he turned the art on its head and thereby created the internal school.” 
Huang Baijia referred to the external arts of Shaolin as emphasizing attack, while the internal arts are able to overcome movement with stillness and propel attackers with a simple wave of the hand . The internal/external distinction is still defined today as such, assuming that the external arts are made for combat and primarily use physical force, while the internal arts are more subtle, use energy, Qi 氣, and focus on the philosophical paths called “self-fulfillment” .
This distinction between internal and external, between the arts of Wudang and the arts of Shaolin, could be more ideological and political than technical. Indeed when Huang Zongxi in 1669, then Huang Baijia in 1676, made mention in their writing of internal and external arts, the Qing had just taken power in China. Stanley Henning indicated that the external arts of Shaolin represented foreign Buddhism, and symbolized the Manchu aggressors (the Qing), while the internal arts of Wudang represented Taoism, and therefore, symbolized the Chinese people. 
Note that this internal/external distinction was the initiative of martial arts practitioners created at Mount Wudang who wanted to stand out from the martial arts in force at the time, most of which were linked to Shaolin.
The three main styles representatives of the internal arts are Taiji Quan 太极拳, Bagua Zhang 八卦掌 and Xing Yi Quan 形意拳.
Nan Quan Bei Tui 南拳北腿, says an old Chinese proverb ; southern fists and northern legs.
This geographic distinction is generally appropriate that northern styles use more kick and southern styles do the same with fists. The Yangtze River, which crosses China from west to east, symbolizes this north/south separation .
The old adage had, however, been very often criticized for being too caricature. For example, Huang Hanxun 黃漢勛, a master of Tang Lang 螳螂, criticized the proverb Nan Quan Bei Tui in his martial arts manual published in 1954  :
Southern boxing arts, spanning the area from Guangdong to the Qinling mountain range [also called the “Southern Mountains”], typically emphasize a constant barrage of strikes. They send out their fists without overly extending their shoulders or elbows in order to prevent opponents from getting control of their arms, which would then make it easy for an arm to get broken by an opponent. Using their fists in this cautious way, they wield their legs even more cautiously. Therefore in southern boxing arts, even if their sets (taolu 套路) have over a hundred movements, there may be only two or three kicks. Because of this, the layman thinks that any kicking is drowned out by all that punching, and thus he only talks of “southern fists”.
Northern boxing arts typically emphasize long-reaching punches, low stances, and kicks which often go higher than one’s own head. Kicks comprise about twenty percent of each set, most of them high, and this strongly catches the layman’s attention, who thereafter proclaims “northern legs”
Gabrielle and Roland Habersetzer recently attempted to explain this distinction by putting forward several arguments  :
In the south, a country of fishing and rice growing, where part of life is spent with your feet in the water, attitudes are more static and combat techniques mainly call on the upper limbs and blocking techniques. Another reason would be historical overcrowding, hence the habit of taking into consideration the limited space one might have to fight in the populous streets.
In the north, on the contrary, country of the great outdoors, nomads and hunters on horseback, Chinese boxing consists more of long techniques, “flying”, see acrobatic, and many dodges that do not take into account a possible obstruction.
This distinction is the most recent. We talk about most often of traditional Wushu and modern Wushu. It appeared following the sportivization of Chinese martial arts in the 1950s by the communist government. Gradually competitions were set up with sporting rules to assess and decide between the competitors. The martial arts presented in these competitions were chosen for their gymnastic and choreographic qualities. From this were born, for example, the terms Chang Quan 長拳 and Nan Quan 南拳.
The term Kung Fu 功夫, although associated with Chinese martial arts, does not represent a particular style. Rather, it defines a concept of accomplishment and perfection through intense, long and committed work. I like this concept. I often tell my students that there is no secret in Kung Fu, there is only work. And the work always pays off. At a time when everything goes too fast and everything is consumed and thrown quickly, this concept is almost a philosophy of life in itself.
As for the classifications of styles of Kung Fu, they are numerous, sometimes obvious but sometimes also hazardous. For my style, Wing Chun 詠春, it is undeniable that it is a southern Chinese style. However, system experts do not always agree on the internal/external classification. And besides, to satisfy everyone, we sometimes talk about hybrid style ! Both internal and external ! One also finds in Wing Chun, the influence of the Chinese “3 teachings” ; Confucianism, Taoism and Buddhism. It is also difficult from this point of view, to position this martial art, in a well-defined “box”.
Finally, for my part, I consider that the term Kung fu is very often used as a misnomer in the West. And I also find it regrettable that some practitioners sometimes do not know what style they are really practicing. I will quote an anecdote to illustrate my point. At a martial arts forum, I met practitioners from a “Traditional Kung Fu” school. That was the term they used. Curious, I asked them exactly what style they were practicing. They simply told me that they were doing traditional Kung Fu. I told them that I also did a traditional style of Kung Fu and that this style was called Wing Chun. In the end, they couldn’t tell me what their style was, or even their current, their influence … and I find it a shame to limit themselves to this term Kung Fu which represents everything and nothing at the same time. Chinese martial arts are very rich and diverse, a minimum of precision seems necessary to me.
 Notice du cong-fou des Bonzes tao-séé, dans Mémoires concernant l’Histoire, les sciences, les arts, les mœurs, les usages, des chinois par les missionnaires de Pekin, tome IV, p442-443, AMIOT Joseph, ed. Nyon l’aîné, 1779. Source : BNF Gallica