CRT, or Cathode Ray Tube monitors, though differing from televisions in several respects, share one important characteristic. Like TVs, the picture you see is constantly being redrawn on the screen. This refreshing action is the cause of the almost imperceptible flicker seen in the typical computer monitor image.
Now, the slower the monitor refreshes itself, the more perceptible this flicker is, and the more perceptible the flickering is, the more likely you are to suffer from eyestrain, headaches and general fatigue as a result of it. Windows XP defaults to a refresh rate of just 60Hz, meaning that the screen redraws itself 60 times a second. This is pretty much the lowest acceptable rate, and higher refresh settings can have a considerable positive effect on computer experience.