Yin and Yang represent the two opposites which make up the universe, and the balance between them. i.e. male and female, light and dark, hot and cold, soft and hard etc.
The light half (yang) represents the heaven and all that is positive: male, light, warm, living and fiery. The dark half (yin) represents the earth and all that is negative: female, dark, cold, soft and deadly.
As nothing is ever completely black or white, both Yin and Yang carry a small piece of the opposing side to show balance. It is the interaction and balance between these two elements that lie at the heart of feng-shui studies.