In Kung Fu there is often talk about Yin and Yang, and with time I've started to appreciate the notation more and more. First I thought it was "humbug" just like I think about many other medicine disciplines, but now I understand more about it.
It's not rocket science, it's actually a very simple concept. All things are in harmony of two opposite sides. No side is more "important" than the other and one side can't exist without the presence of the other. For example, there can't be a front if there isn't a back.
This site explains it very well. I read all of it. What shocked me though was the on the "Supreme Ultimate" symbol, Yang is the white part and Yin is the black part. I've always imagined that it was the other way around. Damn!
"Yin and Yang may be the most important theory in Chinese Medicine"
I also found this nice page about Qi Gong which I will read now.