When to create a new app (with startapp) in Django? Ask Question

I've googled around for this, but I still have trouble relating to what Django defines as "apps".

Should I create a new app for each piece of functionality in a site, even though it uses models from the main project?

Do you guys have good rule of thumb of when to split off a new app, and when to keep functionality together with the "main project" or other apps?