RGB are the colors red blue and green they are the additives colors. Mixing these colors will give you a white color. Computer screens yse these to prouduce these by emitting light through these color.

CMYK are the colors cyan magenta and yellow to make black.  They  are subtractive colors. This is a hard to print because the  all printing inks contain some impurities & the end result tends to be a brownish color that must be combined with black to form a true black color. Because of this, not all colors that can be mixed on the computer in RGB mode can be made in real life.

http://www.printernational.org/rgb-versus-cmyk.php