Tai Chi belongs to the Chinese martial arts practices although the term martial arts is in my opinion a little misleading because Tai Chi is an activity in which one moves very slowly and deliberately which is very easy on the joints. The basic idea is to bring the body into balance and improve body control, posture and breathing. In addition it helps to reduce stress.
In Tai Chi there are many different forms that kind of glide into one another in defined patterns and almost look like a very elegant dance from the outside. While performing those movements a specific breathing pattern is performed.
Later, Chinese weapons will also be included.
The focus is on posture and slow flowing movements which usually start from a central point, like the hips and then work as a unit together with arms and legs.
Qi Gong exercises are often mixed with the Tai Chi exercises but more centered on meditation, breathing and relaxation. Qi Gong is supposed to balance the Qi in your body and lead to more energy.