A color wheel is a circular diagram in which primary and usually intermediate colors are arranged sequentially so that related colors are next to each other and complementary colors are opposite.
The first color wheel has been attributed to Sir Isaac Newton, who, in 1706, arranged red, orange, yellow, green, blue, indigo, and violet into a natural progression on a rotating disk. As the disk spins, the colors blur together so rapidly that the human eye sees white. From there the organization of color has taken many forms, from tables and charts, to triangles and wheels.
Check out this excellent post on ColourLovers Blog that tells all about the many different types of color wheels.