The App.config file is used in .NET applications, particularly in desktop applications built with technologies like Windows Forms, WPF, or console applications. It serves a similar purpose to the Web.config file in ASP.NET, but it's specific to non-web applications. The App.config file is an XML-based configuration file that allows developers to store various settings and configurations for an application.

The Web.config file is a configuration file used in ASP.NET web applications. It plays a crucial role in defining various settings and configurations for the application. Here are some of the primary purposes of the Web.config file:Application Configuration: The Web.config file contains settings that configure the behavior of the ASP.NET web application. This includes settings such as custom error pages, authentication modes, authorization rules, session state configurations, and more.Connection Strings: Database connection strings are often stored in the Web.config file. This allows developers to centralize database connection information and easily update it when necessary.Custom Error Pages: Developers can specify custom error pages for different HTTP status codes in the Web.config file. This helps in providing a more user-friendly and informative experience when errors occur.Security Settings: Security-related configurations, such as authentication and authorization settings, can be specified in the Web.config file. This includes specifying which users or roles have access to specific parts of the application.Application Modules and Handlers: The Web.config file can be used to register HTTP modules and handlers. Modules are components that can participate in the processing of requests, while handlers are responsible for processing specific types of requests.Compilation Settings: Developers can configure compilation settings in the Web.config file, including whether the application should be compiled dynamically or precompiled, and other compiler-related options.Session State Configuration: Developers can configure how session state is managed in the Web.config file. This includes specifying whether session state should be stored in-process, out-of-process, or using a custom provider.Custom Configuration Sections: Developers can define custom configuration sections in the Web.config file to store application-specific settings in a structured manner.URL Redirects and Rewrites: The Web.config file can be used to define URL redirects and rewrites, allowing developers to configure how URLs are processed and mapped to specific resources.Tracing and Debugging: Tracing and debugging settings can be configured in the Web.config file to assist developers in diagnosing issues during development.The Web.config file follows an XML-based format and is located in the root directory of an ASP.NET web application. It can be edited manually or modified through Visual Studio and other development tools.