"web.config" is scoped to a particular directory. You use it to override machine.config for a particular application or application sub-folder. Sorry for not answering sooner; there is an ASP.NET sub-forum that you might want to use for these types of questions in the future.

web.config is configure ur apllication only
but machine.config file related to ur all d application on ur server.
the settings made in the web.config file are applied to that particular web application only whereas the settings of machine.config file are applied to the whole asp.net application.
The MACHINE.config file contains default and machnine-specific values for all supported setting.
Machine setting are normally controlled by the system admin and app should never be given write access to it.
An application can override most default values stored in the machine.config file by creating one or more web.config files.

At min an app creates a WEB config file in its root folder. The web.config file is a subset of machine.config written according to the same XML achema.