To expand upon what happymat just told you, I'd like to share a little trick I learned just recently.

When you hold "v," you can hold down your MMB (middle mouse button) and gently "gesture" near the vertex you want to snap to, the vertex you have selected will snap to the one you gestured near. This you know; happymat just told you :p

But for example, what if you didn't want to snap directly TO the non-selected vertex? What if you wanted to snap ONLY in one direction; X, Y or Z?

Easy solution. With your move manipulator selected, click on the arrow corresponding to the direction you want to move your vertex in (X, Y or Z), and it will turn yellow. Now if you MMB gesture, as above, your vertex will move ONLY in the axis you specified (the arrow that turned yellow).

Pretty sweet, eh?

EDIT:

This does not apply to vertex snapping only; you can do it with grid snapping as well.