This is the Taijitu (太極圖), with black representing yin and white representing yang.  It is a symbol that reflects the inescapably intertwined duality of all things in nature, a common theme in Taoism.  No quality is independent of its opposite, nor so pure that it does not contain its opposite in a diminished form: these concepts are depicted by the vague division between black and white, the flowing boundary between the two, and the smaller circles within the large regions.
From File:Yin yang.png, converted to SVG by Gregory Maxwell.
Gregory Maxwell