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, as you know, 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'.
Now, since 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 confusing people too much, 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 talks about Qi Gong, unless they state otherwise you can probably assume that they are talking about something that has to do with their energy, and their bodies. Qi Gong in it's more abstract forms, such as (Neo-) Confucianism, is a bit less commonly heard, and many people also do not place practices such as philosophy and spirituality under Qi Gong; they prefer using Qi Gong to solely refer to the healing/medicinal aspect of the practice, an example of this would be Tao Yin.