Kung refers to any training or study that requires a real effort over a sufficient period of time. Chi Kung refers dealing with CHI over a long period of time. Literally, CHI means Air. Chi Kung originally refers to the practitioners of breathing exercises. From breath, the meaning expands to the breath of life or the life force. It is the energy of life.
The Taoists looked at life from a different perspective than westerners. They explored the Life Force Energy. They found that they could use their minds to learn to control their Energy (CHI). The Taoists combined the breath and mind and found new ways to control the Life Force Energy. They found ways to move the Life Force Energy with their mind alone.