The yin yang symbol is an important concept in Chinese philosophy, science, medicine, and martial arts. The symbol signifies two opposing forces that are still dependent and connected to each other. The symbol can be traced back to the third century BCE. In Chinese cosmology, the universe formed itself from chaos and the yin and yang was used to form life and the physical world around us. In modern times the yin side, black side, symbolizes femininity, water, moon, and nighttime. The yang side, white side, represents masculinity, sky, sun, and daytime. During Confucius times in ancient China the symbol was used to enforce a strong patriarchal society.
I often see the yin and yang symbol in jewelry and clothing designs. I typically see it being used to express an Asian or even psychedelic feeling. I rarely see the idea brought up outside of an academic setting.