RGB vs. CMYK
(a basic fly-by)
The RGB (Red, Green, Blue) color model is used in all CCD (digital) cameras and scanners as well as computer monitors, tv's and projectors
The CMYK (Cyan, Magenta, Yellow, Black) color model is used by all toner- and ink-based printers to represent colors and images with various sized dots of these 4 colors.
If you print an RGB image to a CMYK printer, you'll typically lose color saturation, and get too much detail carried in the black ink. This results in an image which has too much contrast, but with flat color.