Short for Cyan-Magenta-Yellow-Black, and pronounced as separate letters. CMYK is a color model in which all colors are described as a mixture of these four process colors. CMYK is the standard color model used in offset printing for full-color documents. Because such printing uses inks of these four basic colors, it is often called four-color printing.
In contrast, display devices generally use a different color model called RGB, which stands for Red-Green-Blue. One of the most difficult aspects of desktop publishing in color is color matching — properly converting the RGB colors into CMYK colors so that what gets printed looks the same as what appears on the monitor.
Recommended Reading: Webopedia’s The Science of Color and the Digital Camera Quick Reference.