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RGB image is for eyes only i would say while CMYK is a type of image that would be used by the Printing studios which use ink to print. You need to work in CMYK mode to get an aproximate feeling of what the printed material may look (you are not seeing the real final product as you are watching the image on a computer screen, the real product can be seen only after printing).
In short: CMYK is subtractive color generally utilized in the printing industry, while RGB is additive color for the most part utilized as a part of showcase representation.
The name of the CMYK stands for Cyan, Magenta, Yellow, and Key (because in four-color printing, cyan, magenta, and yellow printing plates are carefully keyed, or aligned, with the key of the black key plate. ).
The name of the RGB color stands for Red, Green, and Blue. The main use of the RGB color is for sensing, representating and display of images in electronic devices such as televisions and computers, though it has also been used in conventional photography.