Perhaps one of the most misunderstood truths when printing in color. The 2 acronyms above refer to the color palette which is used in graphic design. By default, most art programs will use the RGB color palette because many photos and web graphics will be using this standard palette. When setting up your files for printing however, you must convert the color pallet from RGB to CMYK as all commercial printing presses use 4 base colors referred to as CMYK printing.
When you request printing in 4 colors, the different hues and shades of color are all derived from a combination of 4 base colors, Cyan, Magenta, Yellow & Black. Many printers will simply convert the color pallet for you before they print your order. Unfortunately, when you convert the color pallet to CMYK sometimes the color looks different than what you see on your monitor. This is the cause of all the horror stories you hear about when here someone say "My brochure doesn't look the way I wanted it to". The safest way to achieve color printing accuracy is to create your artwork using the CMYK pallet from the start. Doing this will give you a better idea of how your colors will look when printed.
Most printing companies will convert your graphics to CMYK in order to print. However if you convert your image yourself you will have a better representation of how the finished product will look.
If you do not convert the files to CMYK yourself, you run the risk of your job looking different than expected because the conversion was not made. Supplying files in CMYK format is the responsibility of the graphic designer.