Stands for "Cyan Magenta Yellow Black." These are the four basic colors used for printing color images. Unlike RGB (red, green, blue), which is used for creating images on your computer screen, CMYK colors are "subtractive." This means the colors get darker as you blend them together. Since RGB colors are used for light, not pigments, the colors grow brighter as you blend them or increase their intensity.
Technically, adding equal amounts of pure cyan, magenta, and yellow should produce black. However, because of impurities in the inks, true black is difficult to create by blending the colors together. This is why black (K) ink is typically included with the three other colors. The letter "K" is used to avoid confusion with blue in RGB.