The CMYK model is a subtractive color model using four colors, Cyan Magenta Yellow and Key (black). The ink is usually applied in the order of its abbreviation, but this can depend on the printer.
The CMYK model works by masking colors on a white background. So white is (0,0,0,0). The ink reduces the light that would otherwise be reflected. This model is subtractive because white is the natural color of the paper or other background, while black results from a full combination of colored inks (100,100,100,100).
To save money on ink, and to produce deeper black tones, unsaturated and dark colors are produced by using black ink instead of the combination of cyan, magenta and yellow.