Tetradic colors are those that form a rectangle on the color wheel, and are also sometimes called rectangular colors.
In a tetradic color set there are two complementary color pairs rotated 60 degrees from each other. Not a description that does well with words so take a look at the image to the right. It makes more sense visually.
Because tetradic colors are made up of complements it tends to be a very vibrant, although possibly jarring, color scheme. Care needs to be taken when using this scheme and it is best if one of the colors is dominant to the others.