The Wheel of Law, or the Wheel of Dharma/Dharmachakra, is a Buddhist concept that uses the wheel of a chariot as a symbol for righteous action that will ultimately lead to the end of the cycle of samsara (rebirth). It is said that Lord Buddha set the Wheel of Dharma into motion with is first sermon at the Deer Park in Sarnath; this is known as the "first turning." Subsequent turnings of Dharmachakra took place during his sermons at Rajgir and Shravasti, respectively.
The wheel itself represents not only the cycle of rebirth, but also karmic action. The wheel's motion is a metaphor for not only Lord Buddha's setting into motion rapid spiritual maturity for an individual seeking enlightenment and freedom, but also the individual's turning of the cycle of rebirth through karmic action or cause and effect. The more "bad karma" one puts into motion, the more likely history is to repeat for that individual, most likely through rebirth into another samsaric realm, or life-form for easier understanding; the more "good karma" one sets into motion, the less suffering he/she will endure in this world...and the next.
On the Wheel of Dharma, there are eight spokes; symbolizing the Noble Eightfold Path that Lord Buddha taught: right view, right intention, right speech, right action, right livelihood, right effort, right mindfulness, and right concentration. The Wheel of Dharma is also divided into three parts or "three trainings" of Buddhist philosophy: the hub symbolizes moral discipline or ethical conduct, the spokes are representative of wisdom while the rim symbolizes concentration or meditation. Action sets the wheel into motion; however, the wheel turns as one unit with all three parts working in co-operation.
By leading a righteous life, one set's into motion good karma and possible re-birth in a higher realm; however, the goal of moksha (liberation or release) being the paramount goal. Moksha is the ending of the cycle of rebirth and suffering. However, wrong or incomplete actions can set the wheel turning negatively; thus creating "bad" karma, which leads to not only more suffering and pain for the individual (and ultimately all of creation since everyone and everything is interconnected), but also re-birth into a lower samsaric realm...which leads to more suffering. By following Lord Buddha's teachings, the Wheel of Dharma can be stopped with the reward of ultimate liberation.
As a symbol, the wheel is used to defeat ignorance, suffering and other negative or otherwise incomplete or "evil" actions, thoughts, desires, etc. However, in order to set the wheel into motion, produce "good" karmic action, and ultimately stop the wheel's cycle; one must completely follow the Lord Buddha's teachings.