http://en.wikipedia.org/wiki/Tai_chi_chuan
As the name "tai chi chuan" is held to be derived from the Taiji symbol (Taijitu or T'ai chi t'u), commonly known in the West as the "yin-yang" diagram, tai chi chuan is therefore said in literature preserved in its oldest schools to be a study of yin (receptive) and yang (active) principles, using terminology found in the Chinese classics, especially the Book of Changes and the Tao Te Ching. The core training involves two primary features: the first being the solo form (ch'üan or quán), a slow sequence of movements which emphasize a straight spine, abdominal breathing and a natural range of motion; the second being different styles of pushing hands (tui shou,) for training movement principles of the form in a more practical way. more ...

http://en.wikipedia.org/wiki/Pushing_hands
Pushing hands is said to be the gateway for students to understand experientially the martial aspects of the Internal martial arts; leverage, reflex, sensitivity, timing, coordination and positioning. Pushing hands works to undo a person's natural instinct to resist force with force, teaching the body to yield to force and redirect it.

Health oriented tai chi schools may teach push hands to complement the physical conditioning available from performing solo form routines. Push hands allows students to learn how to respond to external stimuli using techniques from their forms practice. Among other things, training with a partner allows a student to develop ting jing (listening power), the sensitivity to feel the direction and strength of a partner's intention. more ...

The term "kung fu" was not popular until the 20th century, thus the word would be seldom found in any ancient texts. The term was first known to have been reported in the 18th century by a Westerner, French Jesuit missionary Jean Joseph Marie Amiot, and was known little in the mainstream English language until approximately the late 1960s when it became popular because of the Hong Kong films, especially those by Bruce Lee, and later "Kung Fu" - the television series. Before that it was referred to primarily as "Chinese boxing".

Translation and Interpretation
It is hard to translate "kung fu" into English as we have no equivalent. In short it means "achievement through great effort" or simply virtue. It combines (kung) [sometimes gong] meaning achievement or merit, and (fu) which translates into man. In Mandarin, when two "first tone" words such as gong and fu are combined, the second word often takes a neutral tone, in this case forming gongfu.

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 endeavour. You can say that a person's kung fu is good in cooking, or has kung fu 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.

For a process to be truly kung fu; motivation, self-discipline and time must be present.