When you setup your Windows VNC server, there are options regarding
"polling" such as "Poll Full Screen" or "Poll Foreground Window".
These options control the manner and rate in which the VNC Server 
sends screen updates to the Viewer, but it's not immediately clear 
what each of the options actually mean. Here are some good 
explanations taken from the VNC mailing list.

From Adam Pavelec [apavelec@benefit-services.com] :

"For VNC 'Update Handling', you can choose to receive full 
screen updates (which is very bandwidth intensive), or you
can choose to receive updates to the foreground application 
window (the window that's in 'focus' -- this i the default 
option in RealVNC), or yo can choose to receive updates for 
/only/ what's underneat hthe cursor.  AFAIK, 'Poll Console 
Windows Only' is especially useful for terminal session and
command/DOS prompt use."

From William Hooper [whooper@freeshell.org] :

Basically WinVNC (note the Unix servers are different) needs 
to "look" at the screen to send it over the network.  The "poll"
options tell WinVNC where to look:

Poll Full Screen       = "Look" at the full screen for changes
Poll Foreground Window = "Look" at the forground (selected) window 
                          for changes, etc.

Illtud Daniel [illtud.daniel@llgc.org.uk] :

Since Windows doesn't 'know' about VNC it won't tell VNC 
when something on the screen changes (unless you're using 
the UltraVNC video driver version). VNC has to poll something
(the videobuffer?) to see if there are any changes to transmit
to the client.

Polling is generally inefficent (since most of the time nothing
changes) and you have a tradeoff between polling frequency (doing
it all the time takes clock cycles) and responsiveness (if you
only poll every second, you're going to have quite a lag), buu
polling is often the only way to accomplish monitoring.