A starting definition: The nature of change, balanced as two halves of a whole.
Yin Yang is perhaps the most known and documented concept used within Taoism.
Yin Yang is the concept of duality forming a whole. We encounter examples of Yin and Yang every day. As examples: night (Yin) and day (Yang), female (Yin) and male (Yang). Over thousands of years quite a bit has been sorted and grouped under various Yin Yang classification systems.
Yin Yang illustrated from the Tao Te Ching 
When people see things as beautiful,
ugliness is created.
When people see things as good,
evil is created.
Being and non-being produce each other.
Difficult and easy complement each other.
Long and short define each other.
High and low oppose each other.
Fore and aft follow each other.
A few basic concepts which define the nature of Yin Yang are:
Nothing is completely Yin or completely Yang. Each aspect contains the beginning point for the other aspect. For example: day becomes night and then night becomes day…
Yin and Yang are interdependent upon each other so that the definition of one requires the definition for the other to be complete.
The nature of Yin and Yang flows and changes with time. A simple example is thinking about how the day gradually flows into night. However, the length of day and night are changing. As the earth ages, its spin is slowing causing the length of day and night to get longer. Day and night are not static entities.
Sometimes changes in the relationship between Yin and Yang can be dramatic where one aspect can literally just transform into the other. As an example: some species of fish have females that transform quickly into males when the population of males aren’t enough.
One effect of this is: as one aspect increases the other decreases to maintain overall balance of the whole.
Four possible imbalances exist:
These imbalances can be paired: so an excess of Yin can also simulate a Yang deficiency and vice versa.
As an example this concept is especially important for Chinese healing practices. So an excess of Yang results in a fever. An excess of Yin could mean the accumulation of fluids in the body. Chinese healing examines a person’s health is in terms of the eight principles: Internal and External stimuli, Deficiency and Excesses, Cold and Heat and Yin and Yang.
For example a Yang aspect of Heat: can be further subdivided into a Yin warm or Yang burning.
Additional material for Yin Yang can be read here:
Now forget everything you have learned about Yin and Yang for a moment. Step back from all the descriptions and classifications of Yin and Yang to consider the following passage from the Tao Te Ching:
The Way begot one,
And the one, two;
Then the two begot three
And three, all else.
A clearer understanding of Yin Yang requires looking back into the Tao. The Tao can be considered as the fundamental absolute. Upon examination: the nature of the Tao expands out. This process of expansion defines a pattern, splitting apart into finer and finer patterns. Yin and Yang is the point where perception demarks the Tao’s expansion into one and then one into two.
Taoism as a practice enjoys examining patterns. Over the years countless sects of Taoism have formed and quite a bit of literature written over the delineation and description of these patterns. Often times Taoists use the concept of Yin Yang as a familiar starting template to work with patterns.
For example Qigong is based upon the patterns of breath and physical movement. Knowledge of bodily patterns forms the basis of this Taoist practice to keep a body healthy. The human body and its movements are divided according to Yin and Yang categories. So the upper body corresponds to the Yang, while the lower body roots into the Yin. The body’s center is where the Yin and Yang meet. Qigong exercises are grouped and explained in terms of Yin and Yang to help classify the body’s harmonies into a working practice. Knowledge of Yin or Yang isn’t required to perform Qigong, instead it’s an additional filter which helps people connect to the practice.
Another example is Taoist divination within the Book of Changes / I Ching. Divination in Taoism is a practice of examining human interactions based on well known psychological patterns. To generate a result either coins or yarrow stalks are tossed down to form a pattern. The patterns generated correspond to Yin and Yang defined qualities. The nature of the Yin and Yang pattern are applied against the psychology of the moment to return advice for the person asking the question.
Yin Yang is a fundamental aspect of Taoist thought. We always naturally apply human based values over naturally occurring patterns. However, remember it’s also important not to chase finer and finer descriptions of these patterns, to do so would be to chase down infinity.
Yin Yang is a system of recognizing how to separate out patterns in our life while also relaxing to accept the overall whole and complete nature of the Tao.