Yin means "shady side," and yang means "sunny side." The two together represent the balance of the two halves as a whole. The symbol represents Taoism.
The Taijitu symbol is commonly known as the yin-yang symbol. It is a half black, half white circle in which the two colors swirl together. It is a concept used within Taoism to represent both sides of a whole. Within the black and white of each side of the symbol is a smaller oppositely colored circle that represents the dependence of each piece on the other. Some examples of yin and yang include night and day or female and male.