RGB or Red, Green and Blue, are additive colors and are what we see when we look at our computer monitors and televisions screens. The tiny dots that make up our displays are composed of RGB information. The RGB color space is very large and is ideal for images that would be used for web and presentation purposes.
CMYK or Cyan, Magenta, Yellow and Black, are subtractive colors are the standard ink colors for printing. This means that whenever we print an image, we are using CMYK inks to produce the print. Many professional printers or publishers require that images for print must be converted to CMYK before being printed. This is because the RGB color spectrum (displays) is much more wide then the CMYK spectrum (ink) and during conversion from RGB to CMYK, the appearance of certain colors may look different.