The monitor is not a stable device. It drifts from its initial parameters over time and you may observe that the way images look on screen is different from the way they look in print. In this case you may need to calibrate you monitor. Calibration is a comparison between measurements - one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device. The device with the known or assigned correctness is called the standard. The second device is the unit under test (UUT), test instrument (TI), or any of several other names for the device being calibrated. So Monitor Calibration is the process where the monitor is placed into some predetermined standard, it can be done either by manually or by using calibration hardware like EFI ES-1000 spectrophotometer which is a handheld measurement device that helps you achieve predictable color every time.
During last decades the most common way for monitor calibration was visual calibration using Adobe Gamma or some other softwares. This method did not always give the required quality of calibration. Each monitor in a network needs to be conformed to the same standard which can be achieved only with a hardware calibrator. Now, particularly in professional DTP, hardware calibrators are standard methods for achieving consistent color which don’t rely on subjective observation. Systems, such as GretagMacbeth's Eye-One Monitor or ColorVision's Monitor Spyder LCD/CRT packages, which measure your monitor with either a spectrophotometer or colorimeter are common on the market.