A web.config file is a Windows file that lets you customize the way your site or a specific directory on your site behaves. For example, if you place a web.config file in your root directory, it will affect your entire site (www.coolexample.com). If you place it in a /content directory, it will only affect that directory (www.coolexample.com/content). However, in order for a web.config file to register in a specific directory, there must be a web.config file in the root directory.
Note: web.config files are XML documents that work on Windows servers. ".config" is not an extension like .html or .txt. For more information on how to set up web.config files, visit Microsoft's Web site.
See Also
Configuration Reference 
