Hard qigong is the cultivation of chi energy and utilising it to reinforce the body to resist physical punishment. Typical training involves deep breathing, high tension movements and repeated striking of the body; gradually building up the force over a long period of time, until the body becomes hardened. Here is an example of hard qigong by the famous Shaolin monks.
Hard qigong should generally be practiced while you are young. It places great stress upon the body, and over a long period of time can cause damage. Many martial artists who practice hard qigong while young, move naturally over to soft qigong.
Soft qigong is more familiar in the west as the slow movements and deep breathing exercises practiced by the older generation in parks across China. It is the cultivation of vital pre-natal chi, that is said to reduce illness and extend life.