Spot color is one that is printed in its own ink. In contrast, process color (4-color) is produced using 4 inks (cyan, magenta, yellow and black). The most common system to specify spot colors is Pantone. If you are printing a logo or image that needs certain specific colors, spot colors should be used. Spot color printing is more cost effective when the artwork contains 4 or less colors.
Process colors are represented by specifying a percentage value of the four colors. When dealing with more colors or photographic images, process color printing is more economical and can produce more true to life results.