Kung Fu is a chinese word which literally means "great skill" or "well done" but is typically used to refer to a vast variety of chinese martial arts.
There are many different focuses within Kung Fu, depending on which particular style is chosen. Mok Gar for example focuses more on kicks and leg strikes than an art like Wing Chun, which focuses more on the upper body techniques. Chin Na, although incorporated into other systems it is actually a full system, focuses on grappling and many aspects of it. One of the main problems within Kung Fu, or rather, between practitioners of martial arts, is what exact style they practice, and what this actually means in terms of what kind of martial artist they are. For someone with experience in what the different styles are like, it's easy enough, but a Kung Fu practitioner talking to a Karateka or Aikidoka, would have to go more in-depth and explain.
The Chinese martial arts, though thousands of years old, still retain their traditional training methods. Of course though, like all martial arts, that is providing you can find a good teacher, something which many people came to believe they could find if they went to China. But that has become the subject of some heavy dispute, because as many people know after the 'Boxer's Rebellion' (which I won't go into), many of the authentic teachers of Kung Fu were executed, and the government took control of all major places of training, leading to a massive-scale watering down of training in China, at least in any big institutes such as the Beijing Academy. The art they now teach is Contemporary Wushu, taught mainly for its display purposes rather than its combat effectiveness.
That's not to say there isn't authentic training though, it's just much harder to find.
There are many differences within the styles of Kung Fu as earlier said, but one of the main differences is between four styles and the rest. Tai Chi, Xing Yi Quan, Bagua Zhang and Liu He Ba Fa, or six harmonies eight methods boxing. These are the highly respected arts that promote good health, longevity and internal energy flow (though each tend to have a different focus, and Liu He combining the other three along with its own focus). These arts are also revered for being somewhat lethal in combat, Tai Chi in particular, which is commonly translated into "supreme ultimate boxing", was designed by acupuncturists to target the energy centres of the body, and affect them adversely - every single movement and position within the forms of Tai Chi are intended to target specific areas, practiced slowly so when sped up, the accuracy and thus the damage would theoretically be very high. But this 'secret' of Tai Chi has not only become less secretive, and also somewhat forgotten or ignored by those who only practice it for the health benefits.