It Contains application data stored in the form of files including MDF files, XML files, text file as well as other data store files. The user account utilized by the application will have read and write access to any of the files contained within the App_Data Folder. By default, this is the ASPNET account. The default ASPNET account is granted full permissions on the folder. If we change the ASPNET account, we have to make sure that the new account is granted read/write permissions on the folder. Various ASPNET application features, such as the providers for membership and roles, as well as the Web Site Administration Tool, are configured to work with the App_Data folder specifically.

Note: As a security measure, files in the App_Data folder are not served by the Web server. Do not store any Web pages in the App_Data folder because users will see an error if they request a page from that folder.