CMYK stands for cyan, magenta, yellow, and key, or black. These are the four colors of ink used in the traditional method of printing hardcopies of images, called offset printing. The three colors, plus black, roughly correspond to the primary colors, from which can be mixed colors across the visible spectrum. CMYK is a color mixing system that depends on chemical pigments to achieve the desired hues.
Before the advent of desktop inkjet or color laser printers, most images printed on paper usedoffset printing with CMYK colors. A color picture is separated into its separate, constituent parts to create four related pictures in cyan, magenta, yellow, and black. Each image is made into a plate onto which the right concentration, or amount, of colored ink is applied. When the four plates each print onto a page, the colors recombine and form the original image. For example, a deep plum might have equal amounts of cyan (blue-green) and magenta (pink), with a tinge of black.
Of course, CMYK cannot reproduce any color that exists in the world, but it can produce a great number. It's impossible to match things like a parrot feather, rose petal, or oak leaf, but the color system can get remarkably close. CMYK is capable of creating so many different colors because we not only use inks in varying ratios to each other, but with a varying concentration, noted as a percentage. These combinations create colors that span the spectrum in hue (what we think of as color) as well as tone, or intensity. It is important to note, however, that CMYK is limited by outside factors including the qualities of the paper, the integrity of the ink, and the halftone dot size.
Subtractive color refers to how light wavelengths interact with the world, and how our eyes interpret those interactions as color. Sunlight bouncing around is basically white light, which includes all the wavelengths, or colors, in the spectrum. When sunlight hits a bright orange traffic cone, the plastic material in the cone absorbs some of the red parts of the spectrum, along with most of the green, blue, and violet. All that's reflected is some red, orange, and yellow that equals hazard orange to our eyes. Thus, some colors are "subtracted," leaving behind the color that we see.
Now that we live in a digital age, much is made of the conversions between CMYK color and RGB, or red-green-blue color. RGB color varies light, instead of pigment, to achieve the visible spectrum. We encounter RGB color in monitors that actually emit light at a certain wavelength, rather than reflect existing light. Therefore, the screen image of a picture in RGB will never match the printed image in CMYK. Although these color systems are related, one color cannot directly convert to another.