CMYK are the four inks used in some color printing: Cyan, Magenta, Yellow, and Key. This is a subtractive color model. The ink is usually applied in the order of the abbreviation but depending on different operators some may do it different. The CMYK model is subtractive because it “subtracts” brightness from the white substrate it’s being printed on. In otherwords the model works by partially or entirely masking colors on a lighter, mainly white, background and the ink reduces the light rather then reflecting it.
Some people suggest that the “K” in CMYK is from the last letter in black because the letter B is already used for blue. Although this is useful for memory it is incorrect. In four-color printing there are plates that are carefully keyed or aligned with the key of the black key plate. The “K” stands for the key that locks these colors together and allows for a wider array of colors.