running a django app, help please?

I'm sort of confused with the django app system.
I own a free hosting, by alwaysdata.net (just to learn the django
stuff), and it says that I can make an app using the following line:
python mysite/manage.py startapp [appname]

When reading the django book (at the djangobook.com), it never talks
about app's really. What I want to know is what is the point of apps,
and how do you use them? Thank you.
-Raj

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