1. What is Chi/Qi? Simply put, it is that which makes anything alive rather than dead. Common terms include vitality, life-force, breath, spirit, and in other languages, prana, pneuma, ruach. Normally we speak of different qualities of it such as in-born or genetic vs. that gained from nutrition or exercise. And clearly the quantities will differ, for example in relation to one’s state of health. Chinese medicine delineates many qualities of chi, but it it not necessary to know them to benefit from Chi Gung exercises.
2. What is the difference between Qi Gong and Tai Chi/Ji? In general, Chi Gung is what gives Tai Chi it’s energy, it’s “juice” and distinguishes it from other dance and exercise forms. It is also a building block/component of Tai Chi, comparable to grammar as a component of writing, or rhythm as a component of music. Structurally, Chi Gung exercises appear in simple sets of say 6 or 7, or individual exercises that can be done repeatedly and continuously for as long as one desires. Tai Chi forms are comprised of one specific movement after another, with some repetitions, in a prescribed order, the total number ranging from 16 to over 100. Consequently Tai Chi requires much more memorization which takes time away from learning the inner workings, so I advise beginners to start with Chi Gung.
3. Will this type of exercise make me lean and trim? Probably not because it works on the internal health of the organs, joints, spine, blood vessels, nerves, etc., rather than externals of adipose tissue and muscle mass, considered to be unreliable indicators of health. But indirectly, as states of balance are achieved, your weight may stabilize to your normal range.
4. Is this eastern tradition a substitute for western exercise? Yes and no. Yes in the sense that it will do everything and more for the respiratory and cardio-vascular systems that modern exercises do, as well as build excellent muscle tone and stamina. But if you are a person who is only really satisfied with more vigorous activity, there is no reason to stop it. With time you can learn to incorporate the east into the west, and into all your activities. That being said, it should also be said that there is no substitute for learning the eastern exercises themselves first, if you want to learn a good integration of both systems.
Here is a longer discussion of the two different orientations: http://qigongtaichimassagesfbay.com/articles-4/health-v-fitness/
5. Are these exercises done by visualization? No more than any normal body function is, such as swallowing, walking, or having sex. The key is the kinesthetic sense; one feels what the body is doing externally and internally. This calls for some practice even if you have natural talents. Since the energy is more subtle than the physical body, finer levels of feeling must be cultivated, which can take months and years.
6. Where does Breathing come in? Usually later in the game but one could start with it. Taoists like to train the mind to directly influence the body and energy, and see the breath and say, hand movements as intermediary steps. But general breathing guidelines and goals are: continuous movement without stopping or holding, smooth transitions between inhale & exhale, neither so full nor so empty as to cause any strain, slowing and lengthening the time of each breath, increasing the lung capacity at a comfortable rate, using the pressures to massage the abdominal organs, getting more blood into the interstitial spaces, calming the nervous system, etc.
Here is a brief description of Taoist Longevity Breathing http://qigongtaichimassagesfbay.com/breathing/