RGB (Red, Green, Blue) is the standard for photography and specifying all colours – in various proportions – to obtain any colour in the visible spectrum.
CMYK refers to the four inks used in a standard “4-colour” printing process (cyan, magenta, yellow and black). The K in CMYK stands for “Key”, The “key plate” is said to add the “detail” to a printed image. This is true in that the black plate in a four color process print pushes the contrast and creates “detail”.
But you do not have to bother about this. Just keep all your files in RGB (because RGB is more extended compared to CMYK) , and in case of offset prints, the printing company will separate your file before going to the press.