Hello,
Everything is an application in Django, and you can see them
in settings.INSTALLED_APPS.
Then you can include your app models/forms/etc. from
anywhere else in the project containing the previous app.
To check if the app is registrered or not, you can use the
Django Application (new in Django 1.7). Please read
https://docs.djangoproject.com/en/1.7/ref/applications/
From your example, you can have an app called "comment"
that inside has some models and can be related to anything
else, or the reverse any of your model in your project related
to some model from the comment app.
For more information, please check out the "comments" app
in django.contrib :)
Thanks
aRkadeFR