Yin and yang are the two opposing forces that drive the universe, according to Taoist belief. These forces are opposite but equal. Yin and yang are not considered to be inherently good or evil, but simply as complementary aspects of reality.
Yin represents earth, darkness, femininity, passivity and reception. Yang, meanwhile, represents heaven, light, masculinity, activity, and creation. Other values and elements are assigned to yin and yang, but these are the core tenants of each. The two parts, as a whole, form the Tao. Rather than work as opposites, the two forces complement one another and exist in relation to one another.
The yin-yang symbol, known as the taijitu, consists of two "S" shaped halves of a circle colored white and black, with a dot of the opposite color in each half. It symbolizes the interlocking, interdependent nature of yin and yang. The presence of the dots represents the idea that opposites are expressed in terms of one another. In other words, darkness is the absence of light and vice versa.
In traditional Chinese medicine, yin and yang are thought to govern the health of the body. When one of the forces is out of balance, it causes sickness. Practices, such as acupuncture, are intended to balance the body's energy levels.