Qi gong pronounced "chee gong" or "chi kung" is an ancient Chinese movement breathing exercise.
Tai chi and qi gong are often practiced together. There are many styles of qi gong. However, all use breathing exercises to increase the amount of energy in the body. The theory used is based on yin and yang.
Qi gong is one of the very few exercised that can actually give you more energy, like food and sleep. Tai chi on the other hand, regulates energy flow but doesn't create more of it.
Practising qi gong has many different health benefits. It helps to boost the immune system, fight off disease, aids in relieving stress and anxiety through its slow moving movements. It is a form of meditative practice.
Qi gong is probably the ultimate martial arts as masters that practice qi gong cannot be touched by anyone wanting to attack them. This isn't seen in kung fu films as it doesn't make for dramatic viewing. But a qi gong master can push their energy from beyond their body which creates a force-field, where no one is able to enter and hit the person.