The dilemma: pleasing colors you see on your monitor may not look the same as those that come out of your printer or match what someone else sees on their monitor. Color calibration allows you to recreate what you see on your monitor, either in a print or on another calibrated monitor, assuming that the other monitor is also calibrated.
Good color management has three components, calibrating the camera, the monitor and the printer. Printer profiling involves using either a colorimeter or a spectrophotometer to create a custom ICC pro- file for your device. Both Datacolor and X-Rite make color calibration devices for this purpose. The practical difference between the two is that X-Rite uses a spectrophotometer and Datacolor uses a tristimulus colorimeter to read the device specific color gamut.
The spectrophotometer has been around for a long time and is used for many scientific light-measuring purposes. The colorimeter is a newer technology that has developed along with digital imaging. Colorimeter technology is similar to that used by digital cameras and is arguably more accurate in a digital workflow. Either way, both systems produce excellent results.