In short, printers and monitors produce colors in different ways.
Monitors use the RGB (red, green, blue) color model, which usually supports a wider spectrum of colors. Printers use the CMYK (cyan, magenta, yellow, black) color model, which can reproduce most—but not all—of the colors in the RGB color model. Depending on the equipment used, CMYK generally matches 85–90% of the colors in the RGB model.
When a color is selected from the RGB model that is out of the range of the CMYK model, the application chooses what it thinks is the closest color that will match. Programs like Adobe Photoshop will allow you to choose which color will be replaced. Others may not.

Additionally, what you see on the screen may be a simulation of a particular output device. These simulations are driven by color profiling, and may be affected by monitor calibration (or lack thereof).

Color Profiles

Adobe products have default color profiles active when installed. These defaults aren’t generic, nor are they universal. In the North American market, the Adobe default profiles are set for web presses. While there are many other applications for web presses, what you’re probably most familiar with is mass-market “junk-mail’, such as supermarket weekly specials & coupon books. If “junk-mail” quality is the results you’re looking for, you’re set with the Adobe defaults. But if you’re looking for something a bit better, you have choices.

A good start is to switch from the default preset bundle and choose “North America Prepress 2“. That will keep many of the same settings, but activate some warning dialogs if multiple and/or mismatched color profiles are being used in the documents and/or linked images.

At Dynagraphics, all of our presses – conventional and digital – are sheetfed (not web). So further refinement would be to change the CMYK Working Space to either US Sheetfed Coated 2 (old but still good) or Coated GRACoL 2006 (ISO 12647-2:2004). Both of these profiles are included with Adobe software, and allow the reproduction of more colors than the default, particularly with neutral-dark colors. GRACoL is newer profile, and is increasingly becoming a standard in the global market.