What You See
As a desktop publisher or graphic designer, developing a newsletter or document for yourself or that of a client, what you see on screen is actually different than what you will see when you print. As mentioned, RGB is what we are used to when we view images on a computer screen and even that of a TV screen.
When you print from something from your computer to your printer or if you send artwork to a printing press company, the image is actually separated in four parts; the same image will then be colored first in black, then cyan, then magenta, and then the yellow ink. This is actually how many digital and web printing processes do their printing. Think of CMYK as an optical illusion, something to trick your eyes into seeing a full color picture.
So how do you go about getting your prints in CMYK if they start out in RGB? And what about the common printer that you find in the store? Do you need special parts?
Luckily, all of your concerns are easily taken care of for you - most of the software programs that utilize the use of images or printing will automatically turn RGB images into CMYK. Popular programs which graphic designers will use are easily able to switch from that of RGB to CMYK, such as many programs in the Adobe Creative Suite (Photoshop, Illustrator, InDesign).
Printers too will automatically switch the RGB photo or image that you are viewing to that of CMYK. Depending on the printer of course, there may additional settings that you can play with or set.