What are Yin and Yang?

Yin and Yang are the effects of the naturally inherit duality structure found in life. Both Yin and Yang are needed to create harmony. That is why the idea of Yin being "evil" or "bad" is incorrect. The only thing evil or bad is disharmony. Disharmony occurs when too much power is given to either Yin or Yang. Just like too much activity or heat can be harmful, so too can too much inactivity or cold, health is found when both are equally present.