Common Location: C:\Windows\Microsoft.NET\Framework\v2.0.50727\CONFIG\machine.config
The Machine.Config file, which specifies the settings that are global to a particular machine.
This file is located at the following path:
[Windows Directory]\Microsoft.NET\Framework\[Framework Version]\CONFIG\machine.config
As web.config file is used to configure one asp .net web application, same way Machine.config file is used to configure the application according to a particular machine.
That is, configuration done in machine.config file is affected on any application that runs on a particular machine. Usually, this file is not altered and only web.config is used which configuring applications.
You can override settings in the Machine.Config file for all the applications in a particular Web site by placing a Web.Config file in the root directory of the Web site.