“In Chinese philosophy, yin and yang describes how apparently opposite or contrary forces are actually complementary, interconnected and interdependent in the natural world, and how they give rise to each other as they interrelate to one another.
Many tangible dualities (such as light and dark, fire and water, male and female) are thought of as physical manifestations of the duality symbolized by yin and yang.
Yin and yang can be thought of as complementary (rather than opposing) forces that interact to form a dynamic system in which the whole is greater than the assembled parts.
Yin is characterized as slow, soft, yielding, diffuse, cold, wet, and passive; and is associated with water, earth, the moon, femininity, and nighttime.
Yang, by contrast, is fast, hard, solid, focused, hot, dry, and aggressive; and is associated with fire, sky, the sun, masculinity and daytime.
Everything has both yin and yang aspects, (for instance shadow cannot exist without light). Either of the two major aspects may manifest more strongly in a particular object, depending on the criterion of the observation.
The yin yang (i.e. taijitu symbol) shows a balance between two opposites with a portion of the opposite element in each section.
In Daoist metaphysics, distinctions between good and bad, along with other dichotomous moral judgments, are perceptual, not real; so, the duality of yin and yang is an indivisible whole.
In traditional Chinese medicine good health is directly related to the balance between yin and yang qualities within oneself.”
Text source: http://en.wikipedia.org/