Chigong, or Chi-Gong, or [Tsi `guŋ] is not gymnastics, or breathing excercises, as some sources may say. It is the art and science of working with subtle energies that has been preserved for us primarily in China. It's applications are health and healing, longevity and emotional well-being, increased physical stamina and other parameters which can be used in, for example, martial arts, sexual practices for boosted pleasure and rejuvenation, but on more advanced levels, it's a path of harmonious living and spiritual ascention, a path leading beyond this world and beyond physical being.
Importantly, many scripts in Chi-Gong have been preserved containing the ancient methodoligies. Also, it had been, at least until some break of the recent centuries, a living and evolving tradition: the Taoist monasteries practicing Chi-Gong had contests, both internally and between the monasteries, in various disciplines of Chi-gong application: healing, martial, etc. Often, the most advanced Chi-gong master was to become the chief priest. Thus, the methodoligies have been preserved and improved. The art has been kept alive and ultimately practical.
There are several characteristics that make Chi-gong stand apart from such paths as Hundu or Tibetan Yoga:
Thorough attention is paid to physical body and conscious control of it, at a different angle than Yoga. The complex of lower energy centers, the so-called Lower Dan-Tyen is considered the foundation and it's control is developed as the ground for other practices.
Also, the working with the physical body involves much of practice for flexibility of the joints, under a practical, usable for daily life angle. Unlike Yoga, which in it's pure form, let's be frank, is the practice of boosted ascention (and any excercises for tonus and well-being in dayly life have been added by the more modern gymnast-practitioners, yet they dilute the meaning of Yoga, on the one hand, and on ther other, they are not as good and don't cover all the required areas as, for example, modern physiotherapy does), Chi-Gong involves a lot of practice to make sure joints move smoothly with a good range, making daily life conscious motion an anjoyable thing.
I've seen cases after Ha= Tha Yoga classes, good ones, esoteric ones, et. etc. , gyus had aches in their back etc. ettc.
Movement. Many of the exercises and practices are not done in a fixed posture, or "Asana", but are performed with physical motion, fluid, harmonious and most times unstrenious.
Another side of it, is that physical movement has effect on the issue and transformation of subtle energies, and Chi-Gong makes use of these interesting effects.
Easy does it. That's right: unstrenious. Although there's a hard and fast subset of Chi-Gong, most of Chi-Gong compared to a regular Ha / T-ha Yoga class for energy development, feels like enjoyment with subtle and harmonious motion, together with payin attention to more subtle things. There seems to be no that traditional Yogic Tapas (bearing of discomfort) throughout the session.
I'm for alternating Yoga and Chi-gong, for many reasons: they are mutually reinforcing; they provide complementary perspectives; they can be practiced in the days of different physical condition, or external influences such as weather and astrological factors; Chi-gong is convenient to practice outside, as it has far less practices while seated or based on the ground, but there are plenty of things to do in a standing position with some movements, all of which is good for an outside practice while wearing clothes of any season.