The document discusses changes to the file structure and configuration in Rails 3. Some key points:
- Rails 3 applications are now pure Rack applications defined in the config.ru file.
- The application object encapsulates configuration, routes, middleware, and initializers. It is defined in config/application.rb.
- Bundler is used to manage gem dependencies defined in the Gemfile. It ensures consistent dependencies across environments.
- Helper methods no longer output JavaScript, instead adding data attributes to tags to allow unobtrusive JavaScript behavior.