CMYK it is a color system whose acronym is formed by the colors Cyan (Cyan), Magenta (Magenta), yellow (Yellow) and black (Black). CMYK (also called color-pigment) is a very used in the printing industry.
In addition to the primary colors cyan, magenta and yellow, black is also used, considered the "key color" as it is essential to define the details of an image (for some, the "K" that is part of the acronym would be the English word "Key", which means "key").
The CMYK system is used for color printing with ink, with the objective of hiding some colors when the background is white, to reduce the luminosity and emphasize the combination of the four colors. CMYK can reproduce all major existing color gamuts. CMYK works through printers and photocopiers to reproduce a large part of the colors in the visible spectrum.
While CMYK is the pigment color, the RGB system (red, green and blue) is defined as the light color, being used in objects that emit light (on television, for example).
In addition to CMYK and RGB, there are other color standards, such as Pantone, a library that has a huge range of cataloged colors.