VNC is a free type of software that allows you to remotely control a computer running the VNC server software. It allows you to move the mouse, type with the keyboard, send files, basically it allows you to do anything that you could do if you were at the computer itself. Especially useful when used in conjunction with Hamachi. My personal favorite flavor is a piece of software called Ultr@ VNC. You can find out more about it here.
Grab my favorite flavor of this software (Ultr@ VNC) here.