CMYK stands for Cyan, Magenta, Yellow, and Key (Black) — the 4 process inks used in full-color commercial printing. Every magazine, flyer, brochure, and poster printed on a standard offset or digital press uses CMYK. The printer lays down tiny halftone dots of each ink in varying densities, and your eye blends them into the full range of printed colors.
CMYK is a subtractive color model: each ink layer absorbs (subtracts) certain wavelengths of light reflected from the paper. Cyan absorbs red light, magenta absorbs green, and yellow absorbs blue. Combining all three at full density produces a dark muddy brown — not true black — which is why a separate black (Key) ink is added for sharp text and deep shadows.
CMYK can reproduce approximately 16,000 colors — far fewer than the 16.7 million of the RGB/HEX digital spectrum. Some colors fall outside the CMYK color gamut: bright oranges, vivid purples, and neon greens cannot be printed with process inks alone. This is where Pantone spot colors fill the gap.