James Bennett, a Django core developer, on Django app design:
The art of creating and maintaining a good Django app is that it should follow the truncated Unix philosophy according to Douglas McIlroy: ‘Write programs that do one thing and do it well.’
In essence, each app should be tightly focused on its task. If an app can’t be explained in a single sentence of moderate length, or you need to say ‘and’ more than once, it probably means the app is too big and should be broken up.
As a general rule, the app’s name should be a plural version of the app’s main model, but there are many good exceptions to this rule, blog being one of the most common ones.
Don’t just consider the app’s main model, though. You should also consider how you want your URLs to appear when choosing a name. If you want your site’s blog to appear at http://www.example.com/weblog/, then consider naming your app weblog rather than blog, posts, or blogposts, even if the main model is Post, to make it easier for you to see which app corresponds with which part of the site.