Bomunsa temple - Wheel of the Law

It symbolizes the teaching of buddha : the Dharma (the Law) by representing the cycle of rebirth. This symbol is also used in Hinduism.
The outer circumference represents the world that turns (and thus keep changing). The 8 spokes are the 8 ways that can lead to enlightenment or the 8 directions of the compass. The hub in the center allows its movement. He who turns the wheel is called Chakravarti (in Buddhism, it is Buddha entering Nirvana).