In the Tai Chi school where I train, the teachers teach us basics of both Tai Chi and Kung Fu - both stem from the same principles, and in fact some of the most high-impact and lethal moves in Kung Fu are Tai Chi movements performed super-fast. We practice them as something called "push-hands".

I thought this Tai Chi stuff was going to be super-easy, but it isn't; it's intense work and after getting home one really feels like one got a workout.

The idea is to learn the usage of Chi within the body; that can be used for either warfare or health practice, or both.

Actually, both systems, Tai Chi and Qi-Gung, work together. Qi-Gung emphasizes the internal usage of energy and more meditative stances (static), while Tai-Chi is, indeed, the external form. But both manifest through slow movement.

More info:

http://www.white-magnolia.com/#taichi_chikung