CMYK are colors for all printed materials, while RGB are colors that will appear on a screen.
CMYK (cyan, magenta, yellow, and key) are the four colors in almost all printers. These colors can be mixed in different ratios to replicate the RGB (red, green, and blue) colors that appear on a computer screen. The colors RGB colors on a computer screen are actually impossible to replicate exactly with a printer, but the CMYK colors come close enough. Overall, it’s important to remember that CMYK is used for all printed material while RGB is used for all material appearing on screens.