This is a basic tutorial in head positions in Portrait Photography. I was inspired to write this because of some confusion that arose out of
this thread.
All about ears
There is a school of though that some photographers subscribe to. It says that in portrait photography, the ears of the subject are a distraction and should be hidden as much as possible. If possible. they should be out of focus, to minimize the distraction. I'll point out to this while describing the positions.
Head on position

Photo by
Nomads: will create
This is what some photographers refer to as the mugshot. Looking straight into the camera, many things can go wrong with this pose. But most importantly, this shows both the ears of the subject and that is its usually ruled out. But there are always exceptions and some photographers make excellent portraits head on.
Seven - Eighths position

Photo by
Nomads: will create
This is the view when the subject's face is turned slightly away from the camera and is looking into it. In other words, you'll see a little more of one side of the face than the other. You will usually still see the subject's far ear, so try to keep it out of focus (if that's a distraction). Move the head a bit more and you can get rid of the far ear (as in the example above)
Three - Quarters position

Photo by
Nomads: will create
In this view, the far ear is hidden from the camera and even more of one side of the ear is visible. Because of the distance from the camera, the far eye will appear smaller than the near eye, hence position your subject so that the smaller eye is closer to the camera (most humans don't have equally sized eyes).
Profile position

Photo by
cobalt123
In the profile, the head is turned almost 90 degrees to the camera and only one eye is visible. There is not much to say here, but its nice if the shoulders are at an angle to the camera in this position.
There is much much more to portrait photography. Even head positions can be accentuated by tilting the head. This tutorial was just to get you started.