Taiji is a form of qigong. Qigong is "energy work" in Chinese. I suspect that most people come to qigong after playing taiji for a while and want to know more. I suspect also that it comes as a surprise to find that taiji is not the mother discipline, but one of many hundreds of daughters of qigong.
Qigong can be simplified (at least as a memory devise) as: meditation, medicine and martial art. The three "m"s. The three "m"s are also three gateways to qigong practice. Some arrive at qigong because meditation is important to them. Perhaps they have practiced Zen breath meditation for a while and then pursue other meditation techniques. The very basis of qigong is breathing. In qigong there are sitting, walking, moving and standing meditation techniques. From all I have read the foundation is being able to stand. There is more to simply standing than meets the eye! Taiji is a form of moving qigong.
Some arrive at qigong through the second "m" of medicine. Qigong is a wonderfully well developed system of complementary medicine that has been practiced in Chine for millenia. But, qigong is not a quick fix. Studies document amazing health and healing results in qigong practitioners for all kinds of ailments, including the "big C." But we are talking practice that takes time. Not a quick surgery, out in a day and a week's recovery. Qigong practice takes months, years, a lifetime of daily relaxation, meditation and gentle movement. Qigong helps the body
to help itself through the movement of energy, releasing of blockages to energy and giving the body its own defenses against illness. Qigong is no guarantee that when the flu season arrives you will not get the flu. But qigong prepares the body to fight off illnesses through its own inner mechanisms. The long term benefits are wonderful for joints, muscles, organs, digestion, circulation, tension and a host of other issues.
The final "m" is martial arts and qigong is of the "internal" variety. It is about energy rather than physical strength. Having tried the "hard" martial arts some discover in the "soft" arts a different, perhaps more holistic approach.
Whatever the gateway, gigong becomes a wonderful practice. It's about lifestyle. I have included links to a couple of books I have found very helpful.