Red, Green, and Blue are "additive colors". If we combine red, green and blue light you will get white light. This is the principal behind the T.V. set in your living room and the monitor you are staring at now.
Additive color, or RGB mode, is optimized for display on computer monitors and peripherals, most notably scanning devices.
Cyan, Magenta and Yellow are "subtractive colors". If we print cyan, magenta and yellow inks on white paper, they absorb the light shining on the page. Since our eyes receive no reflected light from the paper, we perceive black... in a perfect world!
The printing world operates in subtractive color, or CMYK mode.
In practice, printing subtractive inks may contain impurities that prevent them from absorbing light perfectly. They do a pretty good job with light colors, but when we add them all together, they produce a murky brown rather than black. In order to get decent dark colors, black ink is added in increasing proportions, as the color gets darker and darker. This is the "K" component in CMYK printing. "K" is used to indicate black instead of a "B" to avoid possible confusion over Blue ink.
One of the most common errors made by inexperienced graphic designers is submitting RGB files. As a result the printer must ask if they would like them to convert to CMYK before we send the files for film output. Most of the time, the color change that will occur is slight. However, every once in a while, the color range after conversion is compressed during the transition to CMYK mode resulting in a complete change in color tones. Be warned that there is absolutely no way to get that deep RGB blue using CMYK, no matter how much we want to.