Rails::Application is responsible for executing all railties, engines and plugin initializers. Besides, it also executed some bootstrap initializers (check Rails::Application::Bootstrap) and finishing initializers, after all the others are executed (check Rails::Application::Finisher).
Besides providing the same configuration as Rails::Engine and Rails::Railtie, the application object has several specific configurations, for example "allow_concurrency", "cache_classes", "consider_all_requests_local", "filter_parameters", "logger", "reload_plugins" and so forth.
Check Rails::Application::Configuration to see them all.
The Application is also responsible for building the middleware stack.
railties/lib/rails/application.rb railties/lib/rails/application/bootstrap.rb railties/lib/rails/application/configurable.rb railties/lib/rails/application/configuration.rb railties/lib/rails/application/finisher.rb railties/lib/rails/application/railties.rb