Qi Gong (氣功, also written as Ch'i kung, qìgōng) is the term used to group all the arts that use Qi exercises, resulting in a plethora of different styles, arts and practices. Because Qi Gong is such a wide term, it's unknown exactly how many styles and practices fall under Qi Gong; estimates range to 3,000 and up.
Translating Qi (氣) Gong (功) gives a little more insight in what this term actually encompasses. Qi, is a very broad term, but here we use it to denote energy. Gong, the second part, is probably best translated as 'work' or 'effort'. Thus, Qi Gong becomes 'energy work'.
As Qi Gong literally means 'Energy work', technically anything that has to do with Qi, is Qi Gong. This would mean that Magick, Psionics, Etheric projection, Wicca (and so on) are all forms of Qi Gong. And in a sense they are; but to keep from causing confusion, Qi Gong almost always refers to any system in the eastern energy manipulation arts and sciences. This includes, but is not limited to, Martial Arts, Buddhism, Taoism, Shinto, (Neo-) Confucianism, Traditional Chinese Medicine, Bushido, and so on and so forth.
In general speech, when someone refers to Qi Gong, unless they state otherwise one can probably assume that they are talking about something that has to do with their energy, and their bodies. This is also the most widely-spread 'form' of Qi Gong, in which it solely denotes a system of breathing and motion exercises that (possibly) benefit the health of the practitioner. An example of this would be Tao Yin.
Qi Gong in it's more abstract forms, such as (Neo-) Confucianism, are a bit less commonly known, and many people do not place practices such as philosophy and spirituality under Qi Gong; how one interprets the term is largely a matter of opinion. On Veritas people use the latter definition of Qi Gong, as a term that encompasses just about all aspects of eastern energy manipulation arts and sciences.