In Chinese philosophy the yin and yang are generalized descriptions of the antitheses or mutual correlations in human perceptions of phenomena in the natural world, combining to create a unity of opposites in the theory of the Taiji. The term liang yi, also known as yin and yang or heaven and earth, has a similar meaning.The dual concepts of yin and yang which describe two primal opposing but complementary principles or cosmic forces said to be found in all non-static objects and processes in the universe. This seemingly paradoxical concept is the cornerstone of most branches of Chinese philosophy, as well as traditional Chinese medicine.
Yīn is the dark element: it is passive, dark, feminine, negative, downward-seeking, consuming and corresponds to the night.
Yáng is the bright element: it is active, light, masculine, positive, upward-seeking, producing and corresponds to the daytime.
Yin is often symbolized by water and earth, while yang is symbolized by fire and air.
Yin (dark) and yang (light) are descriptions of complementary opposites as well as absolutes. Any yin/yang dichotomy can be viewed from another perspective. All forces in nature can be seen as existing in yin or yang states, and two produce constant movement/force of the universe.
source: wikipedia
