Computer monitors often have a "maximum refresh rate" listed in their technical specifications. This number, measured in hertz (Hz), determines how many times the screen is redrawn each second. Typical refresh rates for CRT monitors include 60, 75, and 85 Hz. Some monitors support refresh rates of over 100 Hz.
The higher the refresh rate, the less image flicker you will notice on the screen. Typically a refresh rate of less than 60 Hz will produce noticeable flicker, meaning you can tell the screen is being redrawn instead of seeing a constant image. If the refresh rate is too slow, this flicker can be hard on your eyes and may cause them to tire quickly. As if sitting at a computer for several hours wasn't hard enough!
To avoid flicker, you should set your monitor to use the maximum refresh rate possible. This setting is found in the Monitors control panel in Windows and the Displays system preference in Mac OS X. While 60 Hz is considered a good refresh rate, some people will find that 85 Hz is significantly better.
The maximum refresh rate is determined by three factors: 1) The rate your video card supports, 2) the rate your monitor supports, and 3) the resolution your monitor is set at. Lower resolutions (i.e. 800x600) typically support higher refresh rates than higher resolutions (i.e. 1600x1200).
If you have an LCD monitor, you may not be able to adjust the refresh rate. This is because most LCD monitors come with a standard refresh rate that is well above the "flicker" point. LCD monitors produce less flicker than CRT monitors because the pixels on an LCD screen stay lit longer than CRT monitors before they noticeably fade.