Judy, Artie,
Just one image. Thanks for the comments.
Artie, Dennis, Diane,
HSV (Hue, Saturation, Value) is a color model or a way of organizing colors by different qualities. HSL (Hue, Saturation, Lightness (sometimes called Luminosity)) is another way of organizing colors as is RGB. I should caution that this is not the same thing as gamut (sRGB, Adobe RGB etc).
RGB depicted as above (I should have included an image) would be a cube. The digital approximation of the upside down cone and double cone has a hexagonal cone shape.
The colors Red, Yellow, Green, Cyan, Blue, Magenta are the hues on the vertices of the hexagon at the widest part of the diagrams.
Those colors are also at the corners of the RGB Cube. If you turn the cube up on a diagonal, with 0, 0,0 at the bottom and 255, 255, 255 at the top (for 8-bits of color) it sort of approximates the HSL diagram (on right).
To a degree the choice between how one uses these models is a matter of taste. I prefer thinking and if the controls are available using them in HSV. The difference isn't in the "number" of colors since they can each represent any color.
There isn't any simple way of describing the various differences in use of the different models. So, I'll end this here. The Wikipedia entry (search HSV color) isn't bad.
Cheers,
-Michael-