The nod of
acknowledgement is a standard among guys as a way to let the person know that you see them, without having to resort to using words. Girls will generally not accept the nod of
acknowledgement and opt for a wave of the hand and/or a smile, perhaps even accompanied by words. The nod of acknowledgement is a wordless
conversation.
The only acceptable response to a nod of acknowledgement is another nod. If you speak, you have broken the point of such a gesture.