What is the use of web.config? Difference between machine.config and Web.config?

ASP.NET configuration files are XML-based text files , each named web.config that can appear in any directory on an ASP.NET Web application server. Each web.config file applies configuration settings to the directory it is located in and to all virtual child directories beneath it. Settings in child directories can optionally override or modify settings specified in parent directories.Machine.config provides default configuration settings for the entire machine. ASP.NET configures IIS to prevent direct browser access to web.config files to ensure that their values cannot become public .
At run time ASP.NET uses these web.config configuration files to hierarchically compute a unique collection of settings for each incoming URL target request (these settings are calculated only once and then cached across subsequent requests; ASP.NET automatically watches for file changes and will invalidate the cache if any of the configuration files change).