bhavacakra (Sanskrit, wheel of becoming). A wheel, commonly known as ‘the wheel of life’, represented pictorially in the Tibetan Buddhist tradition as a wheel divided into six sections depicting the nature of life in each of the six realms of rebirth (gati). The six realms are laid out around a central hub around which a cock (desire), pig (ignorance), and snake (hatred) chase one another. Around the circumference is a rim divided in twelve sections depicting in symbolic form the twelve links of Dependent Origination (pratītya-samutpāda). Behind the wheel and grasping it firmly (thus symbolizing that the whole of saṃsāra is within his power) stands Yama, the god of death. According to some accounts, the wheel represents a mirror that Yama holds up to a person at the moment of death. The mirror reflects the possible realms of rebirth, and the dying soul will be drawn to one of them in accordance with its karma.

A Dictionary of Buddhism, Oxford University Press, 2003, 2004 (which is available in electronic version from answer.com)