Tai Chi theory
Tai Chi is based on the theory of yin and yang. In fact, in Chinese Tai Chi is the name of the yin-yang symbol.
The Tai Chi symbol as we know it is very old, perhaps 1000 or more years (@12); the ideas it encapsulates are much older. The ideas of two complementary opposites making up a whole are visible in the world around us; men (yang) and women (yin), day (yang) and night (yin), sun (yang) and moon (yin), to name but a few obvious examples.
Yin and yang transform into each other, hence the dot in the centre of each. Everything waxes and wanes, grows and declines. The goal of practice (although defined essentially by the individual) is to find balance, that central pivot around which everything turns.
Tai Chi videos and documentaries
BBC Trust Me, I'm A Doctor
Trust Me, I'm a Doctor examines how tai chi can affect the body positively.
Tai Chi documentary featuring Grand Master Eddie Wu
An old video of Wu Tai Chi form