Monitor calibration is a process where the colors that you see on your monitor are perfected so that they accurately represent the image that is being displayed (as the picture was taken or modified). There are essentially 3 ways to calibrate your monitor:

Do nothing. This isn't actually a calibration method, but this is what most people do (myself included).

Use a visual monitor calibration technique. There are quite a few of these available on the market. Many are free or nearly free. This approach is better than nothing, but remains very subjective. These programs present a series of screens with colored boxes and gray scales and ask you to make the closest match. However, human response to color is not an exact science. Visual techniques are inherently subjective and therefore inconsistent and inaccurate, but typically better than nothing.

Use a sensor-based monitor calibration technique. A company named ColorVision offers calibration products called Spyder™ and SpyderPRO™. The Spyder is a sophisticated optical sensor that measures colors displayed on the screen. It compares the measured colors with an industry standard known as CIELAB and makes adjustments to the graphics video card. These adjustments "calibrate" your monitor for accurate color display. The Spyder software (either PhotoCAL or OptiCAL) also creates an ICC monitor profile that contains the data from this calibration. This profile is automatically stored in your system where it is used as a default in "ICC aware" applications like Adobe® Photoshop®, Adobe Photoshop Elements, and Adobe Album. The Spyder basically replaces your eyes with an analytical technique that is not subject to caffeine, alcohol, or mood swings. While the Spyder is not free, it is very affordable. The Spyder eliminates the guesswork from the process and produces accurate and consistent results from calibration to calibration.

Calibrating your monitor will not guarantee that the photos that you print out of your printer will match the colors that the camera took -- maybe. It is best to use inks and papers along with the 'canned' profiles supplied by your printer manufacturer. These products are not inexpensive, but if you're serious about taking pictures and want them to look great, it's an investment well worth the money. If you're spending a thousand bucks on camera equipment and then skimping on the actual output of your photos you should pick another hobby.