Chi kung literally means the training of breathing. Not just breathing in oxygen and breathing out carbon dioxide. For that we do not need to learn! The training of breathing in chi kung essentially means the training of our breathing muscles. For what purpose and how to do it form the gist of the training of chi kung.
Chi kung practitioners are not the only types of people that need to train up or condition their breathing muscles. Sportsmen in general need to train up their breathing muscles to get enough oxygen during the vigor of their sports. Opera singers need a pair of powerful lungs, and so do windwood or brass instrumentalists. The focus of chi kung practitioners is more direct, and actually can be applied to all of the above, and of course vice versa!
A student of chi king trains his breathing muscles to be the powerhouse to move chi (or an internal sensation) to travel to every single part of his body. To help this formidable task, there are several stages of training, each requires a high-focused mind, focusing on such internal sensation - for pedagogical sake or for the sake of convenience, it is called chi. As Lao Tz spoke of Tao - I have no word for it, let's call it Tao.
One important stage is the opening of our shoulder and pelvic joints. The opening of which can open blockages that may hinder chi flow and they can then act as "pumping" house for more powerful and more manageable chi flow. This stage has many similarities with the training of sportsmen. Hence some chi kung exercises are similar, in form if not in totality, to common stretching exercises and some others to body building exercises of our core.
Another important stage is to better connect our key breathing muscles, our diaphragm to other parts of our body, in particular making our diaphragm the "mind" or "directing organ" of our whole chi kung practice. This stage has some similarities with the training of singers and windwood/brass instrumentalists. It is interesting to note that a number of chi kung schools include the practice of "voicing" (like sounding the vowel /i/) in their training practice.