The idea, from a philosophical standpoint, is to aid in the development of human potential as well as help one to discover their personal true nature. It heavily relies on the concept of Chi, which is a fundamental energy flow. There are numerous versions of the practice, since they have had many millennia to be developed. Meditation in the Taoist and Buddhist doctrines has long promoted Qigong, and Confucianism has considered it to be a method to develop both personal moral character and one's longevity.
One of the reasons that they are so well known is because of the mass migration of Chinese Diaspora around the world. This allowed many people worldwide to view the practice from a first hand point of view. Increased globalization and tourism in the Far East have also played a part in the spread of Qigong, but one should not confuse genuine meditative concepts with simple gimmicks. Sadly, there are those that have developed sketchy systems and tried to profit off of ancient concepts. However, a decent level of diligence should always be used anyway when evaluating these sorts of things.
Tai chi ch'uan is slightly different, and is comprised of a specific style of internal marital arts. Naturally, this Tai Chi practice also has integrated meditative concepts that make up the fluid motions that it demands of people who engage in it. Naturally, any of these practices present unique challenges and rewards to those who use them as part of a greater part of their meditation. Indeed, despite the fact that the concept of Chi sometimes comes under fire, there is very little controversy over the benefit of properly executed relaxation and exercise motions from the art of Qigong.