By Samsara (bhavachakra) we are talking of all existences that are conditioned by: ignorance, suffering and the unexplainable flow of time, often represented by Yama holding the wheel of life. Nirvana, on the other hand, represents the world unaffected by negative emotions, which by definition is the nature of happiness.
The 12 Interdependent Factors
The Wheel of Life, contains twelve unique images each linked to one another called dvadasanga pratityasamutpada or the 12 Interdependent Factors. These can be found on the outer most circle, each image has its own symbolism to help us better understand Samsara.