Computer monitors emit color as RGB (red, green, blue) light. Although all colors of the visible spectrum can be produced by merging red, green and blue light, monitors are capable of displaying only a limited gamut (i.e., range) of the visible spectrum.
Whereas monitors emit light, inked paper absorbs or reflects specific wavelengths. Cyan, magenta and yellow pigments serve as filters, subtracting varying degrees of red, green and blue from white light to produce a selective gamut of spectral colors. Like monitors, printing inks also produce a color gamut that is only a subset of the visible spectrum, although the range is not the same for both. Consequently, the same art displayed on a computer monitor may not match to that printed in a publication. Also, because printing processes such as offset use CMYK (cyan, magenta, yellow, black) inks, digital art must be converted to CMYK color for print.
Converting Your Artwork to CMYK
In Photoshop or other photo manipulation programs:
Covert color mode to CMYK. (Image>Mode>CMYK)
In Illustrator or other vector art programs:
Create in CMYK color mode.
If you need to do color corrections, do them in RGB mode - it's a larger gamut which means it won't clip colors. Use the CMYK preview and the info palette to look at the numbers, and use the levels command replacing colors to get it looking how you want. A little change is normal, CMYK is not as bright as RGB. The colors you typically will have to deal with are usually blues and oranges. For correcting both - reduce the magenta percentage.
The following link also contains some good information about RGB and CMYK that you may find helpful too: