Yin and yang are terms used to describe qualities and functions in relation to each other. Yin and yang can be described as opposites or relative opposites. The symbol for yin means the shady side of a hill while yang represents the sunny side of the hill. Yin connotes darkness, coolness, moisture, resting, receding, descending or sinking. Yang on the other hand represents sun or brightness, warmth, dryness, activity, outward, ascending or rising.  All things are both yin and yang. For example, if you ask if the ocean is yin or yang, the answer would be "both". The aspect of the ocean, which is deep, dark, and still, is yin. It's aspect of motion with rolling waves rising and crashing is yang. If you ask if a hill is yin or yang, it depends which side, or which aspect. Relative to a plain, a hill is yang, but in relation to a mountain, a hill is yin.  Yin and yang are polar opposites but they are what they are only by relativity to the other. They are dependent on each other as you cannot describe darkness without relative light and you cannot describe depth without shallow. Warm is only yang compared to cold, because it is yin compared to hot. A man who is quiet and withdrawn can be said to be yin next to a woman who is active, outgoing and fiery.  And the point of these descriptions is only to describe balance or lack thereof. If a person has an illness with fever, his yang is overactive and consumes his yin. To restore balance to this person it is necessary to subdue the yang and to support the yin. A disease where an organ is under active would be considered yin such as hypothyroidism. Hyperthyroidism or overactive thyroid is called yang. When the yang and the yin are in perfect balance, there is no dis-ease.  Yin and yang are continuously and naturally transforming into each other. For example, inhaling and exhaling, being  active and resting, eating and excreting, nourishing and cleansing. The natural rhythm of the body can be described as a continuous exchange of yin and yang.  
The Chinese symbol for yin and yang depicted here is described thus. The dark represents yin; the white represents yang. But neither one is absolute. There is always at least a bit of yin within the yang and a bit of yang within the yin. The small black circle within the white shows the yin within the white yang. And the small white circle within the black represents the yang, which is always found within the yin. And the thinning tail of each moving into the other represents the constant transforming of one into the other.

‘No two of the products of creation are alike. From this we know that although the number of things is infinite, at bottom there is nothing without yin or yang [which differentiate them]. From this we know also that the transformations and changes in the universe are due to these two fundamental forces.’