Yin n Yang

In Chinese Philosophy, yin and yang are generalized descriptions of the antitheses or mutual correlations in human perceptions of phenomena in the natural world, combining to create a unity of opposites.

The dual concepts of yin and yang (or heaven and earth) describe two primal opposing but complementary principles or cosmic forces said to be found in all non-static objects and processes in the universe.

The image below (scroll down plz) showing yin-yang is a circle (presenting taijitu- the initial unity of universe) with two parts: white part presents yang and black part presents yin. Two parts pass through each other on a line because yin and yang are never separated, such as if people do not know what bad is, they do not know what good is. There is a small black round in white part and so is in black part; that presents the philosophy: "yang in yin, yin in yang" (for example: though water is fluid-yin, water is also hydraulic-yang); if yin is not in yang, yang is not yang and so on (everything have two aspects). Neither white part nor black part is a semicircle because there is never absolute balance between yin and yang. There is always a stronger aspect and a weaker aspect; that presents the philosophy: "Whenever yin is stronger, yang is weaker and so on". If the circle is divided into two by any diameter, black or white colour never cover all of the area of a segment because universe is never in all yin or all yang.
On the opinion of stay, two parts are put together in a circle; that is unity. On the other hand, on the opinion of motion, two parts are contradict aspects, they fight and interchange each other; whenever the trend of yin increases, the trend of yang decreases and so on. At the specific time when yin is extreme, yin starts to grow into yang and so on (for example: day after night, night after day). The unity maintains universe and the contradiction is the stimulation of the universe's development.

A simple way to put it: In good there is always a little bit of evil, in evil there is always a little bit of good - everything has two aspects.