I am assuming you are refering to RGB as in a computer screen.
RGB stands for red, green, blue which are the spectrums of light used in displays. Combination of Red Green & Blue create the entire visual spectrum available on a computer display from white to black and all colors in between.
While CMYK is used in the printing process Cyan (Blue), Magenta (red), Yellow and Black. These 4 colors are combined to reproduce the spectrum in print.
Hope this helps