import settings will import the first python module named settings.py found in sys.path. Usually (in default django setups) it allows access only to your site defined settings file, which overwrites django default settings (django.conf.global_settings).
So, if you try to access a valid django setting not specified in your settings file you will get an error.
django.conf.settings is not a file but an object (see source) making an abstraction of the concepts, default settings and your site-specific settings. Django also does other checks when you use from django.conf import settings.
You can also find it in the django docs.
Hope this helps.