Every Ember application is represented by a class that extends
`Ember.Application`. This class is used to declare and configure the many
objects that make up your app.

As your application boots, it creates an `Ember.ApplicationInstance` that is
used to manage its stateful aspects. This instance acts as a container for the
objects instantiated for your app.

Essentially, the `Application` *defines your application* while the
`ApplicationInstance` *manages its state*.

This separation of concerns not only clarifies the architecture of your app, it
can also improve its efficiency. This is particularly true when your app needs
to be booted repeatedly during testing and / or server-rendering (e.g. via
[FastBoot](https://github.com/tildeio/ember-cli-fastboot)). The configuration of
a single `Application` can be done once and shared among multiple stateful
`ApplicationInstance` instances. These instances can be discarded once they're
no longer needed (e.g. when a test run or FastBoot request has finished).