Difference between CMYK & RGB
CMYK stands for Cyan, Magenta, Yellow and Black (Key). This color mode is used for physical printing on both small and large format. These colors make up your prints when printing in 4 color process. It is the color space for print material. The four colors combine with varying degrees to produce a physical image, known as subtactive mixing. Everything starts as white and each layer of ink reduces the initial brightness to create the color. Adding CMY together creates the Key color (black). To name a few examples where CMYK are used is in physical printing. This includes promotional products, advertising, branding and .ai & .eps, pdf files.
RGB stands for Red, Green and Blue. This color combination is used for anything displayed on screen. Light source within the device creates the color you need by mixing the three colors and varying their intensities. This is refered to as additive mixing, meaning all colors start as black. Red, green, and blue lights is added to each of them to brighten. All three lights combined will create white. Uses for RGB include web design, social media, and visual content. The best formats to use with RGB are .jpg, .psd, .png and .gif.