ApplicationController is an abstract Controller from which all other controllers inherit. All controllers decend from this super class.
It is an empty controller to which you add filters that should be run on all controllers and methods that should be available to all controllers.
It comes with an Application Helper as well, where you can place methods that all templates should have access to.
Before Rails 0.9.0, the ApplicationController was known as AbstractApplicationController (AbstractApplicationController, itself, was introduced in Rails 0.6.0).
category: Glossary