The Yin Yang or Tai Chi is a Chinese symbol. This symbol was derived from ancient Chinese efforts to understand the patterns in the sky associated with the changing of the seasons. An eight foot tall pole would cast a shadow of varying lengths as the seasons passed. This showed a cycle between times dominated by day and times dominated by the night. Because day and night are opposites, the universe was interpreted as a place of neverending change. This was a result of the interaction and balancing of the opposite forces.
This symbol has grown to teach many that there can be some bad side-effects resulting from good actions (and reverse).