CYMK (Cyan Yellow Magenta Black) is the colour system with which most of us are familiar. It describes colours formed by light reflected from a subject.
Light begins white and parts of the spectrum are subtracted from the white when light hits a subject and reflects back to our eyes. We see Yellow because the subject absorbs all other colours of light and reflects only Yellow. Something Black absorbs all (actually most) of the light and reflects very little. This is the system we use when mixing paints or printing our pictures. Unfortunately, CYMK has a much smaller gamut than RGB. That's why what you see on the screen often seems more vivid that what appears in print.