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ASP.NET defines an application as the sum of all files, pages, handlers, modules, and executable code that can be invoked or run in the scope of a given virtual directory (and its subdirectories) on a Web application server. For example, an "order" application might be published in the "/order" virtual directory on a Web server computer. For IIS the virtual directory can be set up in the Internet Services Manager; it contains all subdirectories, unless the subdirectories are virtual directories themselves.
Each ASP.NET Framework application on a Web server is executed within a unique .NET Framework application domain, which guarantees class isolation (no versioning or naming conflicts), security sandboxing (preventing access to certain machine or network resources), and static variable isolation.
ASP.NET maintains a pool of HttpApplication instances over the course of a Web application's lifetime. ASP.NET automatically assigns one of these instances to process each incoming HTTP request that is received by the application. The particular HttpApplication instance assigned is responsible for managing the entire lifetime of the request and is reused only after the request has been completed. This means that user code within the HttpApplication does not need to be reentrant.