CMYK and RGB color

RGB color, red, green, and blue light are added together in various ways to reproduce a broad array of colors. The main purpose of the RGB color model is for the sensing, representation, and display of images in electronic systems, such as televisions and computers, though it has also been used in conventional photography

CMYK refers to the four inks used in some color printing: cyan, magenta, yellow, and key black.

Graphic Designers need to understand what this is and how it effects them when designing.

http://en.wikipedia.org/wiki/CMYK_color_model
http://en.wikipedia.org/wiki/Rgb