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The refresh rate (or "vertical refresh rate", "vertical scan rate") is the maximum number of frames that can be displayed on a monitor in a second, expressed in Hertz.
The scan rate is controlled by the vertical sync signal generated by the video controller, ordering the monitor to position the electron gun at the upper left corner of the raster, ready to paint another frame. It is limited by the monitor's maximum horizontal scan rate and the resolution, since higher resolution means more scan lines. Increasing the refresh rate decreases flickering, reducing eye strain. On smaller monitors (14") few people notice any change above 60-72 Hz. On larger monitors (17", 19") most people would experience mild discomfort unless the refresh is set to a more comfortable 85 Hz or higher. 100 Hz is comfortable for most people.
When LCD shutter glasses are used for stereo displays, the effective refresh rate is halved, because each eye needs a separate picture. For this reason, it's usually recommended to use a display capable of at least 120 Hz.
Refresh rate has greater effects on CRT monitors, while flicker is less noticeable on LCD monitors, because LCD displays don't have a refresh per se.
Different operating systems set the default refresh rate differently. Windows 95 and Windows 98 set the highest possible refresh rate. Windows NT and it's descendant Windows 2000, however, by default set the refresh rate to the lowest supported, usually 60 Hz.
Old monitors could be damaged if a user set his video card to a higher refresh rate than supported by the monitor. Nowadays most monitors would simply display a notice that the video signal uses unsupported refresh rate.
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