Jonathan,
Harold has got it right. However, on Vatsim some ARTCC's use (for lack of a better term) a radar hand-off at the TWR level. Most TWR facilities in the real world would not have this capability, but as far as Vatsim, it is what it is. The TWR track on Vatsim is really not a TWR radar track. It is some ARTCC's way of performing IFR releases (different from the IFR clearance) between the TWR and Tracon controller.
An IFR clearance lets the pilot know he is cleared on the route. But he cannot takeoff without an IFR release as well in most cases. This is simulated on Vatsim with the TWR tagging said aircraft. When the Tracon controller accepts the hand-off, the Tracon controller has more or less let the TWR know that said aircraft is okay to be released.
Some ARTCC's use this method, others do not. Hope this helps clear up some confusion.
Remember, the Tower for the most part is a visual entity. In the real world. bigger airports may have ground survellience radar, but other than that, a good set of binoculars is what many tower operators use.
Cheers!