Trichromats - Humans have three kinds of color receptor cells, or “cones” -  in their eyes. Each type of cone contains a different visual pigment. These three cone types are called "red", "green" and "blue." Therefore we are “trichromats” (tri = 3, chroma = color).

Tricromats - Some animals, for example bees, have three types of cones. Two of the cones are sensitive to "human visible" wavelengths. The third cone is sensitive to colors in the ultraviolet range of the spectrum.
This cone enables them to see colors that humans can’t see.

Tetrachromats - Most bird species (that have been studied) have at least four types of cones. They are "tetrachromats." Recent studies have confirmed tetrachromacy in some fish and turtles. It has been suggested that some women are also tetrachromats. One study suggested that 2-3% of the world's women may have the kind of fourth cone that gives a significant increase in color differentiation.

Dichromats - Many animals have only two kinds of cones in their eyes. They are known as “dichromats.” It’s worth noting that the color-sensing pigment in these cones may be weak. Therefore, an animal - for example a dog - probably sees very weak colors.

[information found on http://www.post-gazette.com]