The circle is an object of nature, an idealization of pure mathematics, and a symbol or framework and has been used to understand and describe the world and the universe. It is considered a symbol of unity, because all the regular polygons are embraced by the circle. It is also the symbol of infinity, without beginning or end, perfect, the ultimate geometric symbol. To earth-centered religions throughout history as well as to many contemporary pagans, it represents the feminine spirit or force, the cosmos or a spiritualized Mother Earth, and a "sacred space".
The wheel symbol, like the circle is a universal symbol of or unity. It symbolizes unity, movement, the sun, the zodiac, reincarnation, and earth's cycles of renewal. Pagans used it in astrology, magic and many kinds of rituals. A pagan is traditionally anyone who isn't a Christian, a Jew or a Muslim. So a pagan according to Abrahamic religions, is everyone else except them. (The sun wheel was a sacred amulet to the Celtic Gauls or Gaels in Europe. Later, christians adopted the form, changing it slightly, so that it became a christ monogram drawn within a circle. This is also the origin of the celtic cross.)
The Wheel of Dharma is a common symbol of Buddhism. Like the wheel of a cart that keeps turning, it symbolizes Buddha's teaching as it continues to spread endlessly. The eight spokes represent the eightfold Path of Buddha. Hindus and Buddhists use the circle image as the basis for the creation of mandalas. The mandala is a sanskrit word meaning 'sand circle'. It is seen as a microcosm embodying the various divine powers or processes at work in the universe and it symbolizes the totality of existence, inner or outer. The Swiss psychologist Carl Jung considered the mandala to be a universally occurring pattern associated with the mythological representation of the self. That is, for Jung, the mandala is the symbol of the self; a symbol of psychic totality.
In Hindu and Buddhist meditations, the mandala is used to raise consciousness. In meditation, the person fixes his or her mind on the center of the "sacred circle." The centre of some mandalas show a triangle with a bindu (dot) inside a circle. It represents the merging of male and female forces. In Native American traditions the circle forms the basic pattern of the Medicine Wheel and plays a major part in spiritual rituals. In Wicca traditions, the circle symbol is used in rituals and incantations. In astrology, it is the 'Circle of Life' and symbolizes the continuence of life. In Taoism, the circle image is found in the 'Yin Yang' symbol symbolizing universal harmony and the unity between all opposites.
As per the Buddhist interpretation of the mandala, as a means and process towards meditation, I have created some circles of my own with my own slant on things, of course. ;)