DPMS (Display Power Management Signaling) is a standard to reduce power consumption in monitors. Typically, both the monitor and the video card must support the DPMS standard in order to receive any benefit from it. DPMS specifies four modes of operation (in order of increasing power savings): "Normal", "Standby", "Suspend" and "Off". Two signal lines, "Horizontal Sync" and "Vertical Sync" provide a method for signaling these four different states to a DPMS monitor.
A good technical resource on DPMS is available at http://webpages.charter.net/dperr/dpms.htm.
Normal means just that -- the monitor is fully powered and on. LCD laptop panels and LCD flat screens use considerably less power than traditional CRT monitors.
Standby is used to describe a very minor power savings level. This setting usually involves blanking the screen by turning off the electron (RGB) gun. However, the power supply is left on and the tube filaments energized. When you need to use the monitor again, the monitor will come back on very quickly. This option requires DPMS monitor and video card support along with enabling DPMS in XFree86. Standby is sometimes referred to as hsync suspend mode since the horizontal sync signal is turned off to signal this power management state to a DPMS monitor.
Suspend is used to describe a very strong low power state. This setting usually involves the same power conservation as Standby however in addition the power supply to the monitor is turned off. This option requires DPMS monitor and video card support along with enabling DPMS in XFree86. Suspend is sometimes referred to as vsync suspend mode since the vertical sync signal is turned off to signal this power management state to a DPMS monitor.
Off usually means just that -- the computer monitor is turned off. Usually, a small auxiliary circuit stays on to monitor the signals from the computer to turn the monitor back on when data needs to be displayed to the screen. Obviously, this keeps power consumption to a bare minimum (if not zero). While the power saving is substantial, to reactivate the monitor may take several seconds. This option requires DPMS monitor and video card support along with enabling DPMS in XFree86. Both the horizontal and vertical sync signals are turned off to signal this power management state to a DPMS monitor.