This article is from the comp.sys.ibm.pc.hardware.video Frequently Asked Questions, by Michael Scott with numerous contributions by others. (v1.0).
[From: Michael Scott (firstname.lastname@example.org) and Bill Nott (BNott@bangate.compaq.com)]
There are two primary measures of the maximum effective pixel addressability and refresh rate that a monitor is capable of. The maximum rate that a monitor can refresh the screen is measured in Hertz (cycles/second) and is called the vertical refresh rate (or vertical scan rate). The horizontal scan rate is the number of times that the monitor can move the electron beam horizontally across the screen, then back to the beginning of the next scan line in one second. Most early analog monitors were fixed frequency, meaning that they were intended to work only at one specific vertical refresh rate (often 60 Hz) and one horizontal rate (often this is expressed as a number of pixels, but this isn't really the same). Most older SUN, SGI and other workstation monitors were of this type. Generally, these monitors are limited in their applications, since they require that the incoming video signal falls within narrow timing specifications.
These type monitors also typically use composite video signals (with sync on Green), so are not compatible with most of today's PC graphics controllers. Also note that even if the composite video signal issue is overcome, there are additional issues related to attempting to use such monitors with a PC. Among these are DOS text mode support, and radiated emissions compliance. See "How can I get a fixed frequency (RGB) monitor to work on my PC?" below.
In part due to the desire to produce more flexible monitors (i.e. fewer different models), the lack of PC SVGA/EVGA/etc video standards, and in part due to recognition of an emerging trend toward higher pixel addressability formats within the computer industry, along with a desire to provide an upward migration path for new customers, vendors started to produce monitors capable of syncing to video signals within a range of frequencies. Such monitors are called multisychronous, or Multisync. Multisync is actually a trademark of NEC's, though it has become a generic term for a monitor which is capable of syncing to more than one video frequency. The meaning of multisynchronous has become somewhat muddled. To truly be multisynchronous, a monitor should be able to sync to any frequency of incoming video signal (within reason, of course). However, many so-called multisynchronous monitors can only sync to a number of discrete frequencies (usually 3 or 4).
If the video signal supplied to such a monitor is within the range of it's deflection circuits, the image will be displayed; otherwise, the image may be either not synchronized, or completely blanked. It is also possible to harm some monitors of this type by applying a video signal outside it's ranges, if protective measures were not put into place by the design. Thus, such a monitor will usually operate at the most common video modes, but may not operate at less common modes. This type of monitor may be referred to as a 'banded' design. A continuous frequency design should operate at any frequency within the specified range.