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). 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).