Kung fu or gongfu or gung fu (功夫, Pinyin: gōngfu) is a Chinese term often used by speakers of the English language to refer to Chinese martial arts. Its original meaning is somewhat different, referring to one's expertise in any skill, not necessarily martial.
Originally, to practice kung fu did not just mean to practice Chinese martial arts. Instead, it referred to the process of one's training - the strengthening of the body and the mind, the learning and the perfection of one's skills - rather than to what was being trained. It refers to excellence achieved through long practice in any endeavor. You can say that a person's kung fu is good in cooking, or that someone has kung fu in calligraphy; saying that a person possesses kung fu in an area implies skill in that area, which they have worked hard to develop. Someone with "bad kung fu" simply has not put enough time and effort into training, or seems to lack the motivation to do so. Kung fu is also a name used for the elaborate Fujian tea ceremony (Kung-fu cha).
There is a curious contemporary twist on this meaning in the hacker culture: there the fu has been generalized to a suffix, implying that the thing suffixed involves great skill or effort. For example, one may talk of "script-fu" to refer to complicated scripting. It is unknown whether this was consciously based on the original, broader meaning of the term or whether it was a simple wordplay on the less general Western notion of "kung fu".
In Japanese, the characters for kung fu (功夫) retain an approximation of their Chinese reading, and are pronounced kanfū (カンフー). Chinese martial arts in general are also referred to as chūgoku ken (中国拳) or chūgoku kempō(中国拳法), which translates literally to "China fist" and "China fist law," respectively. (Kempō is a generic term for a punching/striking art of Chinese origins.)
In Korean, kung fu means Chinese Martial art but a word that sounds similar is gongbu (공부), which means "study."