Symbolism of Yin Yangs
Yin is the black side with the white dot in it, and yang is the white side with the black dot in it. The relationship between yin and yang is often described in terms of sunlight playing over a mountain and a valley. Yin (literally the 'shady place' or 'north slope') is the dark area occluded by the mountain's bulk, while yang (literally the 'sunny place' or 'south slope') is the brightly lit portion. As the sun moves across the sky, yin and yang gradually trade places with each other, revealing what was obscured and obscuring what was revealed.
Yin is characterized as slow, soft, yielding, diffuse, cold, wet, and passive; and is associated with water, earth, the moon, femininity, and nighttime.
Yang, by contrast, is fast, hard, solid, focused, hot, dry, and active; and is associated with fire, sky, the sun, masculinity and daytime.
The Yin-Yang symbol, having no "officially standardized" rendition, has been the basis of much artistic variation, most of it "frivolous" (that is, "merely" artistic, with no philosophical/mystical meaning, e.g., tattoos).
Yin and yang also applies to the human body. In traditional Chinese medicine good health is directly related to the balance between yin and yang qualities within oneself. If yin and yang become unbalanced, one of the qualities is considered deficient or has vacuity.
Source: Wikipedia Yin Yang
For more information about the History of Yin Yang,