CMYK is the 'Colourspace' used by Offset, Ink-Jet, Laser, Dye-Sublimation, and Thermal printers.

CMYK is a 'reflective' or 'subtractive' colourspace, whereas RGB (Red,Green, Blue) used by Computer Monitors is an 'additive' colourspace.

The CMYK colourspace contains less variations and subtleties than RGB, and therefore converting RGB files to CMYK files causes colour shifts. Every printer is different - colours change depending on the inks, humidity, temperature, paper substrate, and numerous other variables.

Photographs should always be viewed on a callibrated monitor. If it's important that the print run is colour correct, special care is taken by the Pre-Press House or Print Bureau to manage the conversion between RGB and CMYK, and proofs are supplied for a 'press check'.