When photographing someone from the neck up, a head shot, use a medium focal length.
If you're using a digital SLR, use a 70mm focal length, or thereabouts.
If your using a point-and-shoot camera, move the zoom to a middle position, and see if the result is pleasing.
By using the above settings, the volume of the person's face will be more natural looking.
If you use a wide-angle focal length, the face will appear to be bulging.
Similarly, if you use a focal length that's too telephoto, the person's face will appear to be flattened.
This advice also applies when you're photographing anything up close.
For example, if you're photographing a Tiffany lamp to sell on eBay, a wide-angle focal length will distort to lamp.
A telephoto focal length beyond those suggested below, will make the lamp appear to be less round.
Use these focal lengths when doing a head shot, or when photographing an object up close.