The acronym CMYK (pronounced as the individual letters: CM Y K) stands for the process colors cyan, magenta, yellow, and black. These four process colors are the transparent ink colors that a commercial press uses to recreate the illusion of a full-color photograph or illustration on the printed page. If you look at any printed color image in a magazine, especially if you look at it through a magnifying glass (a "loupe"), you will see separate dots of ink in each of the four colors. These four colors, in varying intensities determined by the dot size and space around the dot, combine together to create the wide range of colors you appear to see.
To get these four colors from the full-color image, the image must be separated into the varying percentages of each of the colors. There are several very sophisticated methods of doing this, and the result is a four color separation.
Desktop color systems and the powerful page layout and art programs are now capable of making four-color separations for us. I can also create a color in my publication to match a color in the photograph. For instance, if I want to print my headlines in the same slate blue as in the model's tie, the computer could separate the headline color into the four different layers (sometimes called "plates") as follows: 91%Cyan, 69%Magenta, 9% Yellow, and 2% Black. The photograph itself would be separated into its variations of CMYK. When these four percentages of transparent ink are printed on top of each other, the colors combine to make the full-color photograph and the slate blue of the headlines, all at the same time.
This is different from spot color, where each spot of color is a separate, opaque ink color out of a can, such as red or blue or peach.