What is the difference between a turban and a shemagh?

My understanding is that shemagh is a long scarf that wraps around the neck and face as well as up over the head.

A turban is headgear and, in the West at least, is pre-tied and sewed together so that is only looks as if it might have been wrapped about the head. It does not cover the face or next. Turbans have been popular with women in the US from time to time since the early 1800's and maybe before.