Types of Nods
A fast rapid nod, especially if the listener is touching his ear or rubs his face, show impatience and urge you to move on or let them do the talking. "yeah I got it, let's move on"
Slow and lengthy nods usually show agreement and understanding, they're not urging you to finish talking. "Take your time I'm listening"
Small nods combined with a smile is an encouraging and bonding signal.
Direction
The head is the center of your sensors and therefore will point itself to see, hear and smell more of what it likes and move away from the things it doesn't like.
So if people don't look at you while they nod it can mean that they're distracted and nod only for show.
OR
It can also mean that they're still thinking about something you said earlier. It's hard to think deeply and maintain eye contact, it's their "processing" time. Don't take it personally and often allow your listener a few moments of reflection, or just clarify your words.
Head Beckoning
It's a quick dip of the head downwards or a little toss of the head backwards, usually to signal recognition. It's a nonverbal way of greetings when shaking hands seems inappropriate or uncomfortable OR a prelude to such physical contact. Just think about it as a substitute for saying "I see you, hi there"
A quick note however: People who use this head nod for greetings do so because they prefer to keep their distance and avoid looking too eager. This way they can establish a formal level to the interaction and maintain their 'cool' facade.