Yin and Yang
According to traditional Chinese philosophy, yin and yang are the two primal cosmic principles of the universe. Yin is the passive, female principle. Yang is the active, masculine principle. According to legend, the Chinese emperor Fu Hsi claimed that the best state for everything in the universe is a state of harmony represented by a balance of yin and yang. Unsurprisingly, legend has it that, according to Fu Hsi, true harmony requires yang to be dominant. It's just the nature of things. In classical Chinese, yin and yang refer to the shady and sunny sides of a hill or valley.
From http://skepdic.com/yinyang.html

The Yin-Yang theory is concerned with:
a) The differentiation of opposite factors in any given system
b) The relationships which exist between these opposite factors.
Pure or absolute Yin (and Yang) does not exist. There is always some degree of the opposite contained in anything that exists, no matter how extreme the degree of Yin or Yang. Yin is complimentary to Yang and vice versa.
There does not exist any antagonism between opposites in Nature. They are always complimentary.
The normal, healthy, functional, durable existence of everything in Nature depends on the mutual enhancement and beneficial interaction of opposite forces.
Day and night; summer and winter; work and rest; man and woman. On one level, when there is an antagonistic relationship between opposites this leads to destruction. However, from the larger perspective, the balance of Nature is always maintained.
Yin nourishes and sustains and controls Yang and vice versa. Each thing depends upon it's opposite in order to exist; thus Yin 'creates' Yang and Yang 'creates' Yin. Opposites also hold each other in balance and exert mutual control.
Everything in the physical world has an opposite. There are no absolutes in the manifest universe. Wherever there is a 'front' there will also be a 'back'; the bigger the front, the bigger the back. Birth is followed eventually by death; economic boom by recession; conquest of nature by pollution and degradation of resources. There are no advantages without some disadvantages; there are no disadvantages without some advantages.
Yin attracts Yang and Yang attracts Yin: The greater the difference, the greater the attraction.
Yin repels Yin and Yang repels Yang: The greater the similarity the greater the repulsion.
Yin gradually changes into Yang and vice versa: Spring into summer, summer into autumn, autumn into winter and winter into spring...
At the extremes of Yin or Yang there is great instability and the change from one opposite to the other is rapid and inharmonious.

from http://asiarecipe.com/yinyang .html

The yin/yang symbol is one of the oldest and best known symbols in the world, but few understand its meaning. It represents the two poles of existence which are opposite but complementary, and which exist in everything. Yin represents everything that is feminine, dark, withdrawn, receptive and passive and things moving down and in. Yang represents the masculine, bright, forceful and expansive, and movement out and up.
According to the philosophy, everything contains both yin and yang - this is why we see a spot of white in the black segment of the yin/yang symbol and vice versa. In fact, as something reaches an extreme it always becomes its opposite. This is illustrated in nature in many ways, but applies to all things: a storm is preceded and followed by a great stillness; cold replaces hot replaces cold in the constant cycle of the seasons; an organisation that is too strict will cause a rebellion when the rules become too tight; a balloon will burst if overinflated.
These examples seem trivial, but a person who can spot yin and yang operating in the world can predict the outcome of events. It also accounts for the Taoist's sense of contentment - he has accepted that things change and evolve constantly, moving first one way and then the other. Thus he does not feel upset or worried when obstacles or hard times come - he knows that things will improve. Nor does he get caught up with riches or power when they come his way. Though he might enjoy them, he knows that they are not permanent, and willingly waves them good-bye when the time is right.
In the internal arts the cultivation of the inner power (chi) can be seen as the yang, while the development of softness represents the yin. Movements in the T'ai Chi forms are constantly expanding then contracting, and practitioners are taught not to put themselves in extreme positions to avoid being caught off balance. The interplay of these forces forms the basis of Taoist philosophy, and much has been written about them in the Taoist classics.

From http://www.taoistarts.net/main .html