When I was a kid, I used to wonder if everyone saw the world in the same way. What if what I called "yellow" looked like green to someone else? Wouldn't it be interesting if everyone's eyes interpreted color differently? Incredibly, that's not too far off the mark in the world of digital photography, since computer displays are not all perfectly calibrated to the same specifications.
Every monitor has its own brightness, contrast, and color levels, and that means a digital photo will look different on every computer you show it on. Imagine working to remove a green cast from your photos only to discover that the green was being generated by your display--after editing, they'll look terrible on any other computer.
Adjusting the colors and brightness of your computer display is, in principle, similar to fine-tuning a television. Think about it: When you bring home a new TV, most of the family probably thinks it looks fine, and for the most part, it is. The colors are all about right, and the picture is perfectly watchable. But someone in the house probably isn't happy with the picture. The colors are "too blue," or the skin tones all look "too red." Maybe the display is too bright, or somewhat oversaturated. You can fiddle with the controls by eye, or get a calibration DVD for more scientific results.
Adjusting your PC monitor is much the same. There are many Web sites offering calibration aids and sample photos that can be useful if you want to tweak your display by hand. You'll find an easy-to-use calibration tool at Photofriday.com, for example. A more detailed, step-by-step monitor calibration procedure is available in the form of a Corel Paint Shop Pro help article. And Norman Koren has written an in-depth article that includes links to colorful test images. If you want to improve your display on the cheap, these are great places to start.
Manual calibration is better than nothing, but you'll get better results if you use a colorimeter--a device that automatically calibrates the brightness and color of your display to a reference standard. Any monitor so calibrated should represent a digital image in the same way, giving you confidence that the edits you're making to your photos are accurate. These gadgets all work more or less the same way: You affix it to the monitor and then run a program that adjusts your display. The process takes just a few minutes, and for best results, you should repeat the process every few weeks (since your monitor's settings will tend to drift).
There are all sorts of colorimeter products available. Among the most popular is the family of Spyders from Datacolor. Though they range in price up to US$250, you can get superb results with even the $96 Spyder2express.
I've been using the Pantone Huey (about $70) for a few months, and it, too is an inexpensive but effective calibration gadget. The Huey is different than most since you leave it on your desk when it's not I use, where it constantly optimizes your monitor's settings based on the ambient light level in the room.