App_Start folder has been introduced in MVC4. It contains various configurations files like as BundleConfig.cs, FilterConfig.cs, RouteConfig.cs, WebApiConfig.cs for your application. All these settings are registered within Application_Start method of Global.asax.cs file.
- BundleConfig.cs – This is used to create and register bundles for CSS and JS files. By default, various bundles are added in this files including jQuery, jQueryUI, jQuery validation, Modernizr, and Site CSS.
- FIlterConfig.cs – This is used to register global MVC filters like error filters, actions filters etc. By default it contains HandleErrorAttribute filter.
- RouteConfig.cs – This is used to register various route patterns for your ASP.NET MVC application. By default, one route is registered here named as Default Route.
- WebApiConfig.cs – This is used to register various WEB API routes like as ASP.NET MVC, as well as set any additional WEB API configuration settings.