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what literally defines a django app?

I have read a few questions about what an "app" is supposed to mean in django, but they go into the general purpose/use of an app, which is important, but not what an app literally "is". For example, I felt my curiosity today when
I deleted a folder that I installed as an app with django-admin startapp, and received a certain error that stackO told me was due to a deleted app name residing in my INSTALLED_APPS. After clearing the name, my app worked again
When making a folder cold (just mkdir, no startapp) in the highest level of a project, when trying to import names from real apps, I have to add my project to the sys.path list to be able to import. After remaking this folder as an app, imports are no longer an issue
I've read questions about this topic that had comments like "OK, I've got a models.py file, so it's an app", and it seems like very few people really understand how an app is started.
My question is,
what leeway do I have to modify the apps django makes? Can I delete all the files that come with it (except init) and make it a cold library with no views and models? Are any files besides init required to function correctly?
What does django do when I run startapp that causes an app to be importable automatically, which effect is not there when I make a folder with an init in it (as I said about needing to add the project path to sys.path within that folder). In other words, what does the django command "startapp" actually do to register an app? The action is in django.core.management.templates.py, I read it today and saw things in TemplateCommand.handle() that refer to app or project names, but couldn't see exactly how it registers them. It imports sys, but searching "sys.module" isn't in the file
If I want to turn an empty directory with init into an app, what do I have to do in the shell to make this change without doing startapp?
Thank you

TLDR: Django apps are just Python packages within your project, and you don't need any file except for __init__.py to import it.
I also experienced this vague explanation of "app" from the Django docs, and it led me to look into what a "web app" in the general scheme was before I could understand the concept of a Django one when I was starting out.
Generally defined, a web application is any program transferred from server to client via a browser. This could be an entire website, a certain component in a website (think captchas, widgets, OAuth, etc.), or a function of a website (such as integration with other technologies, like exporting a page to a PDF). These can be modular components or not, portable or not, and distinct within the project's source code or mixed with other things.
Since the general "web app" definition is quite ambiguous, it may be easier to imagine the "Django app." Your project contains one or more apps, of which some may have files or not. Technically, your app can contain nothing except the __init__.py and still be imported (it's just a normal Python package, albeit a useless one). You can make it a library with other modules, but this seems unpythonic, and I make a point to separate business logic from my website source code whenever possible.
Apps are simply things that do something for your project. A question that many people, including myself, like to ask to define an app, is "what does it do?" If you can't answer this question in a concise manner (that doesn't include "and") then your app could be broken into several different parts. This is recommended for your sake, but you may break this rule if you really want. In my first Django project, I made the entire website inside one app folder. It became a nightmare to manage, but I did it.
Views and models are just places to store the information that you need to use for that app in one place; if there was just one big app for your entire website, things would get cluttered and unmanageable very quickly. That would certainly be unpythonic, and the Django developers are very conscious of making things "correctly."
As for technical specifics, Django projects are packages. You can play around with manage.py startproject and checking your sys.path before/after. Apps are packages as well, which (supposedly) contain modules that allow your "app" to perform its intended function. You can use them for anything, everything, or nothing at all, but they are just a Python package with modules in a neat little folder on your system to do something for your project.
You can find a quick overview of applications in the Django documentation here, if you haven't already. Also, this is all a product of my base understanding, so if there is any problem with any part of my answer, please let me know.

Modules not found in django

I want to make an application in django with two apps named apps and data.the "data" apps is placed within "apps".I had entered 'apps.data' in the Installed apps in "settings.py".when I run the devserver i got this error "no modules named apps.data".Any one please help me.

what is the reason for nesting "data" app in "apps"? it is uncommon to nest one app inside of another when there are only two apps (unless you have some great reason to). suggestions:
create an apps app, and create a data model for it;
create an apps app, and a data app; (link them however through the models)
the answer to your problem is probably file structure, but your basic requirements for what you're asking is detailed in your app requirements.
Long story short.. what application are you trying to create?

Django: When doing an include of URL's, should I specify project.app or just app

When including an app's URL's in the project's urls.py, my coding partner does it this way:
('^stops/', include('stops.urls'))
However, Django documentation specifies the following syntax:
('^clients/', include('project_name.app_name.urls'))
His way has worked. Is there a reason to specify the project name at all?

It depends on your PYTHONPATH setting and the structure of your projects and apps.
We have many, many projects. Each with several apps. All are on our PYTHONPATH, so the project name is essential.
If you have only one project, and the top-level project directory is on your PYTHONPATH, then each app can be resolved separately and you can't use the project name.

It also depends upon if the app is inside your project, or a reusable one.
I have a fresh virtualenv for every project, and use a separate mercurial repository for each app. These are then installed into the system path (either in editable form for development, or in non-editable form for deployment), meaning I have <appname> on the PYTHONPATH.

if on the shell you run
import this
you will see that there is a zen of python ''explicit is better than implicit'' hence thats the reason to specify the project name.

Django sub-applications & module structure

I am developing a Django application, which is a large system that requires multiple sub-applications to keep things neat. Therefore, I have a top level directory that is a Django app (as it has an empty models.py file), and multiple subdirectories, which are also applications in themselves.
The reason I have laid my application out in this way is because the sub-applications are separated, but they would never be used on their own, outside the parent application. It therefore makes no sense to distribute them separately.
When installing my application, the settings file has to include something like this:
INSTALLED_APPS = (
...
'myapp',
'myapp.subapp1',
'myapp.subapp2',
...
)
...which is obviously suboptimal. This also has the slightly nasty result of requiring that all the sub-applications are referred to by their "inner" name (i.e. subapp1, subapp2 etc.). For example, if I want to reset the database tables for subapp1, I have to type:
python manage.py reset subapp1
This is annoying, especially because I have a sub-app called core - which is likely to conflict with another application's name when my application is installed in a user's project.
Am I doing this completely wrongly, or is there away to force these "inner" apps to be referred to by their full name?

You are doing it the right way, since django itself does it that way. The admin app for instance is registered in INSTALLED_APPS as django.contrib.admin, but to reset it you have to use manage.py reset admin, and indeed, manage.py reset django.contrib.admin does not work.
It could be considered as a bug in django...
However, you should not be concerned by name conflicts, because you should always run django inside a virtualenv environment, isolated from the rest of the python installation. This is an immensely more powerful and flexible solution than running django on an ordinary python installation. More info, for instance, here: http://mathematism.com/2009/jul/30/presentation-pip-and-virtualenv/

Large Django application layout

I am in a team developing a web-based university portal, which will be based on Django. We are still in the exploratory stages, and I am trying to find the best way to lay the project/development environment out.
My initial idea is to develop the system as a Django "app", which contains sub-applications to separate out the different parts of the system. The reason I intended to make these "sub" applications is that they would not have any use outside the parent application whatsoever, so there would be little point in distributing them separately. We envisage that the portal will be installed in multiple locations (at different universities, for example) so the main app can be dropped into a number of Django projects to install it. We therefore have a different repository for each location's project, which is really just a settings.py file defining the installed portal applications, and a urls.py routing the urls to it.
I have started to write some initial code, though, and I've come up against a problem. Some of the code that handles user authentication and profiles seems to be without a home. It doesn't conceptually belong in the portal application as it doesn't relate to the portal's functionality. It also, however, can't go in the project repository - as I would then be duplicating the code over each location's repository. If I then discovered a bug in this code, for example, I would have to manually replicate the fix over all of the location's project files.
My idea for a fix is to make all the project repos a fork of a "master" location project, so that I can pull any changes from that master. I think this is messy though, and it means that I have one more repository to look after.
I'm looking for a better way to achieve this project. Can anyone recommend a solution or a similar example I can take a look at? The problem seems to be that I am developing a Django project rather than just a Django application.

The best way that I have found to go about this is to create applications and then a project to glue them together. Most of my projects have similar apps which are included in each. Emails, notes, action reminders, user auth, etc. My preferred layout is like so:
project/
settings.py
urls.py
views.py
...
apps/
emails/
urls.py
views.py
...
notes/
urls.py
views.py
...
...
apps:
Each of the "apps" stands on its own, and other than a settings.py, does not rely on the project itself (though it can rely on other apps). One of the apps, is the user authentication and management. It has all of the URLs for accomplishing its tasks in apps/auth/urls.py. All of its templates are in apps/auth/templates/auth/. All of its functionality is self-contained, so that when I need to tweak something, I know where to go.
project:
The project/ contains all of the glue required to put these individual apps together into the final project. In my case, I made use heavy of settings.INSTALLED_APPS in project/ to discern which views from the apps were available to me. This way, if I take apps.notes out of my INSTALLED_APPS, everything still works wonderfully, just with no notes.
Maintenance:
This layout/methodology/plan also has long-term positive ramifications. You can re-use any of the apps later on, with almost no work. You can test the system from the bottom up, ensuring that each of the apps works as intended before being integrated into the whole, helping you find/fix bugs quicker. You can implement a new feature without rolling it out to existing instances of the application (if it isn't in INSTALLED_APPS, they can't see it).
I'm sure there are better documented ways of laying out a project, and more widely used ways, but this is the one which has worked best for me so far.

You should take a look at :
Django generic relations
Django reusable apps best practices if you want to re-use
GIT or any other CVS (git is great for maintaining + deployment)
Fabric if you need automated deployments/updates
I usually use this project structure :
/djangoproject
/apps
/main # the main code
/static # each sub app can serve statics
/app1
/static # each sub app can serve statics
/app2...
/scripts # manage.py, wsgi, apache.conf, fabfile.py...
/core # your libraries ...
settings.py
local_settings.py
Each app in /apps have an urls.py thats autoincluded in the main urls.py. And each app can be a git submodule (or svn external)
Also, using git, you can work on different parallels branches (master/dev/customerA/customerB...) and merge updates.
Creating real reusable is not so easy with django.

You can extract the common functionality into a separate module and make your apps depend on it:
my_portal
auth_module
profiles_module
application1 (depends on auth_module)
application2 (depends on auth_module and profiles_module)
I think the fact that a 'classical' Django project appear to 'contain' the apps it's using prevent you from seeing the picture - in fact, it's not necessary. For a project where you're going to have some sort of pluggable modules I'd suggest organizing the apps as eggs and using zc.buildout+djangorecipe to manage everything.
This way you'll be able to keep your modules in a flat one-level structure. Eggs have the ability to specify dependencies, so if you install application1 (see above), auth_module will be installed automatically.
Also it'll be easy to have different configurations deployed to different servers. Suppose, you have server1 which has application1 installed and server2 which has both application1 and application2 installed - you can just have two configs:
server1.cfg:
[buildout]
extends = base_deployment.cfg
eggs += application1
server2.cfg:
[buildout]
extends = base_seployment.cfg
eggs += application1
application2
djangorecipe also allows you to specify different settings files for each buildout config so you'll be able to add the necessary bits to the main project's urls and installed apps settings.
Not to mention, you can also have a separate config for development configuration (with debug=True and Django Debug Toolbar installed, for example).