The web.config file is a text file which contains XML elements that affect the behavior of an application. It is efficient to store application-level configuration settings in XML-based files that are cached at runtime. Every ASP.NET application contains its own Web.config file , which resides in the root of the application.
The Web.config file overrides many of the settings in Machine.config, within the scope of the specific ASP.NET application where the Web.config file resides. The default Web.config file created by Visual Studio contains comments specific to the most commonly used settings. These comments provide guidance for using the available parameters for a given configuration element . ASP.NET enables you to manage the entire security configuration from the Web.config file.