yin-yang is the basic classification of the material world in ancient Chinese thought. Everything can be assigned to either yin or yang. Yang pertains to masculine, stong, positive, upward properties, while yin pertains to feminine, weak, negative, downward properties. They are ever opposing, but also complementing each other, the existence of either one being the prerequisite of the other. Without yin,t her would be no yang; and without yang, there would be no yin.
yin-yang is also a basic element of traditional Chinese medical theories. it is used to explain the integrated, organic structure of the human body, its physiological functions and pathological changes.