Spot color is specially mixed ink that is applied on the printing press, as opposed to a mix of the four inks, which make up process printing. Spot colors can be produced in a much more vibrant range of colors and can have special characteristics which aren’t available in process inks, such as day-glow or metallic ink. Because they only use one screen, spot colors can reduce the cost of printing if you limit your ink choices to black and one spot color. For example, if you choose to print a red and black logo in process inks, you will actually need three inks on three screens: magenta, yellow and black.
A second reference to spot color concerns the way it is printed in the design. A spot color does not have any gradient or half tones; it only has spots of color in solid form. There is no shading or blending of colors. It is simply one block of ink. Obviously, most standard fonts would be printed in spot color as well as a lot of basic logos and vector graphics.