Yin and Yang
The "yin" and "yang" are the two aspects in Daoist religious and later Daoist alchemical belief that exist in everything in the universe. Both originate from the "Dao" (The Way or Existence), and from the innate balance gave birth to the Three, from which everything originates.
Chinese theory says that everything in the world can be identified as yin and yang in relation to one another. For instance, a light shower is more yin than a patch of sun, but is less yin, and more yang, than a storm. The basis of much of Chinese science in the pre-modern era was about the changes of yin and yang, and the Daoist religion is based around the concept in many senses.
The Yin and Yang diagram shows a dark side (yin) and a light side (yang) each separate but flowing with the other, each containing a small amount of the other (shown as a "dot" of the opposite color).
Yin represents: dark, secret, passive, receptive, yielding, cool, soft, and feminine.
Yang represents: clear, bright, illuminated, evident, active, aggressive, controlling, hard, and masculine.
Neither side is seen as good or evil, both are considered necessary, the preferable state is when yin & yang remain in balance.
Yin and Yang are divided by the line which represents Qi, which is life force, the essence of being.