A way of arranging the colors. Used in art. The color wheel looks like this: (represented using abbreviations for the colors)

         Y
     YG / \ YO
    G  /   \  O
   BG /     \ RO
     B-------R
      BV   RV
         V

Y = Yellow     YO = Yellow-Orange
R = Red        RO = Red-Orange
B = Blue
               RV = Red-Violet
O = Orange     BV = Blue-Violet
V = Violet
G = Green      BG = Blue-Green
               YG = Yellow-Green

The color wheel traditionally has yellow at it's top, and the other primary colors (red and blue) spaced equidistantly. Between the primary colors are the colors you can create by mixing paints of the primary colors, including the secondary colors and the intermediate colors.

Also, you can use the color wheel to idenitify "special" pairs of colors. Colors opposite each other on the color wheel are called contrasting colors and usually make grey when they are mixed. For example, violet and yellow are contrasting colors. Colors next to each other on the color wheel are called analogous colors. Red-violet, red, and red-orange would be an example.

Note that the color wheel does not show tints and shades. Also, colors like red-violet are named with the primary color first.