RGB vs CMYK

RGB
Red, Green, and Blue are "additive colours". 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 colour, or RGB mode, is optimized for display on computer monitors and peripherals, most notably scanning devices.

CMYK
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 colours, but when we add them all together, they produce a murky brown rather than black. In order to get decent dark colours, black ink is added in increasing proportions, as the colour 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.
Always deliver your digital images in CMYK-mode!
One of the most common errors made by inexperienced graphic designers is submitting RGB files. As a result we must ask if they would like us to convert to CMYK before we send the files for film output. Most of the time, the colour change that will occur is slight. However, every once in a while, the colour range after conversion is compressed during the transition to CMYK mode resulting in a complete change in colour tones. Be warned that there is absolutely no way to get that deep RGB blue using CMYK, no matter how much we want to.

Information taken from http://www.copy-cd.biz/info/company.jsp
MJK Disc Productions is a full service provider of CD duplication, DVD duplication, CD replication and DVD replication since the mid 90's.