VNC is remote control software which allows you to view and fully interact with one computer desktop (the "VNC server") using a simple program (the "VNC viewer") on another computer desktop anywhere on the Internet. The two computers don't even have to be the same type, so for example you can use VNC to view a Windows Vista desktop at the office on a Linux or Mac computer at home. For ultimate simplicity, there is even a Java viewer, so that any desktop can be controlled remotely from within a browser without having to install software.
Note: You need to request a license to RealVNC (even for the free version). The free version provides basic remote control for individual private use. It contains no session encryption and none of the premium features contained within Personal and Enterprise.