(vnc stands for Virtual Network Computing) is a graphical system designed to access remote machines: protocol-based RFB (Remote Frame Buffer),
allows you to remotely control a machine. So, as guaranteed by the Remote Desktop feature of Microsoft Windows systems,
it is possible to act on a remote computer as vnc if it were local, acting on your desktop (and managing the components)
through the use of mouse and keyboard.

The vnc system is based on two distinct components. On the one hand you have the server,
need to be installed on your computer that you want to take remote control,
the other a client (or viewer) to be installed on the machine from which you want to access (and control) and remote.
You must also remember that vnc uses TCP ports, with the standard configuration, ranging from 5900 to 5906 (corresponding to as many remote screens).
The following steps illustrate how to establish a connection between a remote computer that uses Ubuntu 10.04 “Lucid Lynx”
(which will act as the viewer) and a remote machine (in this case, on a LAN) with Microsoft Windows XP SP2.

More detail from here introduction vnc
More info at introduction vnc