C.M.Y.K stands for Cyan, Magenta, Yellow, and Key (Black). These represent the four additive secondary colors. Its used in the color printing, and also used to describe the printing process itself. The CMYK model works by partially or entirely masking colors on a lighter, usually white, background. The ink reduces the light that would otherwise be reflected. Such a model is called subtractive because inks "subtract" brightness from white.
R.G.B stands for Red, Green, and Blue. These represent the three additive primary colors. The main purpose of the RGB color model is for the sensing, representation, and display of images in electronic systems, such as TV's and computers, though it has also been used in conventional photography.