What is the difference between RGB Color and CMYK Color ?

RGB: red, green and blue is an additive color space. If you mix the three base colors (red, green and blue) you get white. That is the model monitors use, if the red light and the green light and the blue light is mixed, it becomes white.

CMYK: cyan, magenta, yellow and black are subtractive. If you mix all, you get black.That model is used by printers. If on a dot are printed all three base colors, it gets darker. But it is somewhat hard to mix a good black, thats why often Black is added to the color mix (thats the K in CMYK).

Simply if you are designing for the screen go for RGB, if you're going to print your design use CMYK.

More information can be found in Wikipedia.