Basically it's a remote-control tool, same idea as Carbon Copy, pcAnywhere, Remotely-Possible, and a whole pile of others. The good thing is that VNC is free. You start up VNCViewer, enter the IP address of the remote computer (and password if applicable), then you see the other computer's desktop. When you move the mouse it moves on the remote computer as well. When you type, the letters appear on the remote computer. It's just like you are there.