The words yin and yang have a long history. They were already found on oracle bones from the Shang dynasty (1600 BC). At that point, yang was associated with sunlight during the day, while the yang referred to the lack of sunlight at night.
In time, these terms came to attain a deeper more philosophical meaning. Yang came to represents the masculine, active participation, light, hotness, dryness, the good and the positive. Yin the feminine, passiveness, darkness, coldness, wetness, evil and the negative. .
Together, as yin-yang, they represented the interplay of these opposites in the world. Their balance ensures the harmony in the cosmos. It was believed that both yin and yang are present in all things and that their interaction causes the variety of things we see in the world.
In the image of this post, we see the first visual representation of yin yang. The full image represents the stages of creation. The first circle represents unity, the second the duality or yin and yang. The third the five elements (water, fire, earth, wood and metal). The fourth male and female and the fifth the 'ten thousands things' that we see around us today.
"The Great World History Book"