RGB stands for red, green, and blue. These colors are optimized for display on computer screens and websites. You create colors by adding varying amounts of these colors. For example if you add 255 luminance levels of all three, you produce pure white. Red, green, and blue are sometimes referred to as primary colors, though if you ask a graphic designer what the three primaries are he or she will say red, yellow, and blue.
CMYK stands for cyan, magenta, yellow, and black. Colors created from them are used in printing. In the CMYK process other colors are produced by subtracting specified amounts of these four colors.