Ok, what is Qigong well... Qigong means "Energy Work" Anything to do with building and balancing your internal energy can be classed as Qi gong.
It is used to generate a higher Qi flow in the body. "Qi" is what the Chinese call Energy, and can be likened to electricity, the stronger current you have, the more powerful and healthier your body will be.
An extra high current during practice, will open up energy Channels "meridians" and clear blockages caused by stress and unhealthy living.
Qigong also balances the Yin and Yang in your body. A yin problem in the body would cause an area to be too weak and soft, a yang problem would cause an area to be very tense and blocked etc. Qi gong will create a good balance between Yin and Yang, making for a strong but relaxed body and mind.
There are two main types of Qigong, one being moving and the other being static, the static ones are especially good for when you're just starting out and can't get as much from other parts of your internal training like Tai Chi. But even when you have learnt your internal martial art to a higher level, it is still very beneficial and complements your other training such as Tai Chi.
The Moving Qigongs are also very good too as even though you have to learn the movements they are generally more basic than that of Tai Chi, and are easier and quicker to learn so you get the benefits much sooner. That's not to say that it's better for health than something like Tai Chi, as they go hand in hand, and it's much more beneficial to train in both, than just Qigong or just a form of internal martial art.