Selects connected vertices if one vertex is clicked on. If the connected vertices were already selected then they will be deselected.
Selection takes place by first checking for the closest vertex in that window, it then finds the closest in the third dimension. In this way parts of a complex model may be selected without difficulty.
Clicking away from a vertex will move the 3D cursor to that point.
Holding down the Ctrl key turns off selection/deselection. Using the Ctrl key enables you to place the 3D cursor in areas where vertices may be present without toggling the selection status.