shift + click --> this selects multiple vertices as said before. it de-selects vertices that have already been selected, too.
ctrl + click --> only de-selects a vertice that has already been selected.
You can select more than one vertice by marque-selecting them, however, if you display vertices on more than one object and try to select vertices on both, you will only be able to select vertices on one of the objects.
Also, the vertices on the pole of a nurbs sphere can only be split apart by smoothing the area. This creates a plump, round, cylinder when done. And you thought the poles were only one vertice, didn't you?