A web.config file lets you customize the way your Website or a specific directory on your Website behaves. For example, if you place a web.config file in your root directory, it will affect your entire Website (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.
web.config files work on our Windows servers.
Using a web.config file, you can control:
web.config files are XML documents. ".config" is not an extension like .html or .txt. For more information on how to set up web.config files, visit Microsoft's Website.