CMYK color model refers to the four inks :
Cyan, Magenta, Yellow, and Key (black).
When a color photo is printed it is literally separated into these four colors and printed one color at a time. These four colors then combine to give you the full color image.
This color model is a subtractive color model. CMYK is a 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.
The RGB color model stands for Red, Green and Blue.
Files in RGB format are intended for use on electronic systems like TVs, mobile phone displays and computer monitors.
RGB color system, constructs all the colors from the combination of the Red, Green and Blue colors. The RGB color model is an additive color model in which red, green, and blue light are added together in various ways to reproduce a broad array of colors.
RGB is a device-dependent color model. Different devices detect or reproduce a given RGB value differently, since the color elements and their response to the individual R, G, and B levels vary from manufacturer to manufacturer, or even in the same device over time. Thus an RGB value does not define the same color across devices without some kind of color management.