Yin and Yang
Everything in creation is categorised into yin and yang (i.e. vital essence & vital energy).

 For example, in the celestial system, the moon is yin while the sun is yang. In the human system, females are yin while males are yang. The interaction of yin and yang induces changes and causes life on earth to proliferate. Yin & yang complement each other. Together, they trigger off changes in the universe and gives rise to abundant life.

Fire and water are the signs of yin-yang, reflecting its basic characteristics. Water is cold, restrictive and is relatively stagnant. It is yin in nature. Fire is hot, stimulates and feeds, and is relatively active. It belongs to the yang category.

Do you know, even the kind of food we eat have either yin or yang nature? Therefore, the choice of our meals has a direct relation on our inner yin-and-yang balance and health condition.

Basic Traits of Yin and Yang
Yin: Passive, descending, internal, has a form, cold, dark, suppresses, yielding.
Yang: Active, rising, external, formless, warm, bright, stimulates, firm.

Yin & yang cannot stand alone, nor can they exist in an unequal state. For example, the right side means there is a left side. Left and right have to coexist. It is a complementary and restrictive relationship. They harmonise with and balance each other.

Yin deficiency or Yang deficiency can affect various part of your body.

Applications in Traditional Chinese medicine
The makeup of the human body is also categorized into yin & yang:

The upper body is yang; the lower body is yin.
The exterior is yang; the interior is yin.
The back is yang; the chest and abdomen are yin.
The heart, liver, spleen, lungs and kidneys are yang; the stomach, gallbladder, intestines and bladder are yin.

When a person's yin & yang are in balance, there is good health.

When the yin & yang are not in balance, there is ill health.

When yin & yang leave the body, death comes.