I was asked this question recently and thought it a very sensible one – given the health orientation of Chi Kung and a similar orientation of most people who come to Tai Chi classes, at least with me. So after some consideration my answer was as follows:
Chi Kung ( energy work ) is about learning to move energy around the body in a way beneficial to health – this energy work involves a high degree of body awareness and the ability to adjust internal tensions, posture and the relationships of body parts both statically and in movement.
Tai Chi – is a form of Chi Kung – but also develops the movement aspect further toward principles such as balance, structure, sensitivity, internal power etc. which are of value both in health and in martial arts.
So in early Tai Chi training we focus very much on good healthy body movement based on Tai Chi principles, which can then be extended if desired into the martial art of Tai Chi Chuan.