A VT is a virtual tabletop - sometimes also known as a VTT. Its where you have a map on a computer and tokens on the map which you can move around. You link up the PC with others over the net which can then share the movements so that a GM or player can move his character token or monster token and everybody gets to see that update. Then theres more stuff on top like voice, chat, dice and other stuff to enable play by internet. So people who live remote, or cant get to a game session can still play.

I write and use one see sig but there are about 40 or so of which about 6 are quite substantial. Another popular one is MapTool which many people on this site use. If you go to the keyword index on my sig there are both of them plus VTT and we have a VTT forum section here too for maps created with them in mind.