The concept of yin and yang originates in ancient Chinese philosophy and metaphysics, which describes two primal opposing but complementary forces found in all things in the universe. Yin, the darker element, is passive, dark, feminine, downward-seeking, and corresponds to the night; yang, the brighter element, is active, light, masculine, upward-seeking and corresponds to the day; yin is often symbolized by water, while yang is symbolized by fire.
The pair probably goes back to ancient agrarian religion; it exists in Confucianism, and it is prominent in Taoism. Though the words yin and yang only appear once in the Tao Te Ching, the book is laden with examples and clarifications of the concept of mutual arising.
Yin and yang are descriptions of complementary opposites rather than absolutes. Any yin/yang dichotomy can be seen as its opposite when viewed from another perspective. The categorisation is seen as one of convenience. Most forces in nature can be broken down into their respective yin and yang states, and the two are usually in movement rather than held in absolute stasis.