CYMK stands for Cyan Yellow Magenta blacK. As if you didn't know that already, but just thought I'd throw that in there. But anyways, I work on printing presses where we use those colors to print with. RGB is used on monitors, which is why it's used most often, but the main difference is the color pallete. RGB images will look slightly different in CYMK color, but it gives an accurate representation (well, almost, but the agency will throw on an icc color profile to meet thier printers standards, but still CYMK color) so that when it hits either the printer, plotter, image setter or whatever the final output device is, it will not have to convert the colors and you will be able to make adjustments to your image prior to the final output so it meets your standards. Personally, I get all of my images off of the cd (I still shoot film) in RGB convert them to CYMK for color correction and then back to RGB. Why? So I can get the blacks really great, as well as the other colors. Plus, just force of habit, I worked in the pre-press dept where that's what we worked with. And since even your home printer prints in CYMK, it helps you get the colors right, but then again, most home printers have an ok RGB to CYMK color converter, but since I'm anal about my color, I just do it like that.
I know that I am one with beauty, And that my comrades are one. Let our souls be mountains, Let our spirtis be stars, Let our hearts be worlds.