A lowered head covers the neck with the chin and hence can be a defensive posture that can occur as a result of any perceived threat (not just physical threat).
Lowering the head also lowers the eyes and hence can be a sign of submission, effectively saying 'I dare not even look at you'. The eyes are typically also lowered here. It can be driven by affection ('you are so wonderful') or fear ('you might hurt me if I look at you').
Lowering the head whilst maintaining eye contact can also be a strong flirting signal, typically by women. It says 'You are superior and I just can't take my eyes off you'. It can also be a sign of defiance or caution, for example when showing respect to an enemy ('You are strong and I do not trust you').
Sometimes, lowering the head is just a sign of exhaustion. The head is rather heavy and a tired person's head will sag.
Lowering the head can be a part of ducking as the person reflexively pulls the head down to avoid a real or imagined hazard. This makes the body smaller and protects the neck.
A single short lowering of the head can be an abbreviated nod. This is a common greeting, perhaps as a small bow. It may also be a signal of power ('I am so powerful people are paying will notice even a small nod'). Again it may be a deliberate concealment, sending covert agreement to a colleague.
When the head is low, raising it may be a sign of interest as the person moved to looking at the point of interest. This is typically accompanied by other expressions of interest such as raised eyebrows.
From a level position, a quick flick upwards can be a sign of query ('What do you mean?').
Raising the head and looking at the ceiling may signal boredom. It may also indicate a visual thinker who is looking at internal images. Another alternative is where a person wants to focus on the sound and is thus averting the eyes in order to concentrate on the sound.
Tilting the head sideways can be a sign of interest, which may be in what is said or happening. It can also be a flirting signal as it says 'I am interested in you!'
Tilting can similarly indicate curiosity, uncertainty or query, particularly if the head is pushed forward, as if the person was trying to look at the subject in a different way in the hope of seeing something new. The greater the tilt, the greater the uncertainty or the greater the intent to send this signal.
A tilted head pulled back tends to indicate suspicion, as the uncertainty of the tilt is combined with a defensive pulling back.
The tilted head exposes the carotid artery on the side of the neck and may be a sign of submission and feelings of vulnerability.
If the head is propped up by the hand, it may be tiredness or an expectation of continued interest ('This is so interesting!').
Nodding up and down signals agreement in most cultures and may well be accompanied by smiling and other signs of approval. A vigorous nodding probably indicates strong agreement, whilst slow nodding may indicate conditional agreement (and so may be questioned if you want full agreement).
Turning the head from side to side usually indicates disagreement or disapproval and may originate in infant refusal of food. Again, speed of swinging indicates strength of feeling. A head tilted down whilst swinging may signal particular disapproval ('I don't even want to look at you').
Alternately tilting the head at an angle to each side can say 'I'm not sure', though in Southern India it means 'Yes'.
Nodding or shaking the head whilst talking is an encouragement for the other person to agree (which works surprisingly often).
Nodding whilst the other person is talking sends approval signals and encourages them to keep talking. Shaking the head shows disagreement and they may either stop and seek your view or redouble their attempts to persuade you.
A nod can be used when emphasizing a point. The may range from a subtle encouragement to agree to a rapid and aggressive tilt.
A short, sharp nod can symbolize a head-butt, indicating the desire to strike the other person (this may be in emphasis or for other reasons).
Shaking the head when saying something positive is a negative signal and may indicate the person does not believe what they are saying.
Rotation of the head in a circle is a relatively rare gesture and may just be the person exercising a stiff neck (if they should be paying attention, this may thus indicate boredom).
Turning the head away removes attention and thus may say 'I do not want to communicate with you'. This can be very insulting as it denies the existence of the other person.
Turning the head slightly to the side points the ear at the other person, perhaps better to hear them. This is usually accompanied by continued eye contact and the hand may be cupped behind the ear.
A slight head turn also puts one eye in the middle of your head as the other person sees it. To make eye contact they thus have to focus on one eye. This can be very disconcerting and this 'one-eye' gaze may be used as an act of dominance (It may also be used in the act of 'giving the evil eye').
A slight rotation on top of oscillation may indicate incomplete agreement or disagreement, for example where a nod has a slight additional side-to-side movement, indicating primary or external agreement but with a certain amount of disagreement too (which may be significant if they feel coerced into agreement).
We tend to point at people and things in which we are interested in some way. Pointing the head and face at another person shows interest in them.
In groups and meetings, you can often see power people as others often look at them. Likewise, the less significant people are not looked at often.
We can also point with a twitch of the head in any given direction. Pointing at a person in this way without looking can be insulting and can be subtle, for example where you do not want the indicated person is being pointed at.
We can touch the head in many places. Touching the face is a common sign of anxiety and people tend to have preferred places they touch or stroke when they are concerned. This is a classic pattern that poker players look for in other players as signs of having good or bad hands.
Covering eyes, ears or mouth may say we do not want to see, hear or say something.
We may touch the side of the nose or stroke the chin when we are thinking, making decisions and judging others.
Tapping the head can be self-punishment and hence signal regret, for example tapping the forehead with the heel of the hand ('I'm stupid!'). Note that, depending on context, this can also be a signal that somebody else is considered stupid.
The head is heavy and when tired we may prop it up, either under the chin or at the side. Boredom makes us tired so propping the head may indicate this. Propping up the head also happens when a person is thinking or evaluating.
In some cultures, the head is considered the part of the body that is most spiritual. Touching the head can be considered wrong in such contexts.
The head often moves during speaking and often to signal submission or anxiety. When it does not move, it may indicate that the person is serious or talking from a position of authority. With a stable head, it is easier to fix others with a dominant gaze. This is a quite significant effect that actors often use. Just holding your head steady can lead to others affording you higher status.
When the head is not moving and the eyes are not focused the person may not be attending outer world, instead being lost in inner thoughts and musings.