Web.config file as it sounds like is a configuration file for the Asp.net web application. There is one web.config file for one asp.net application which configures the particular application. Web.config file is written in XML with specific tags having specific meanings.
The initial web.config file for an application must be placed in the root directory of the web application. Sub-directories within the web application can contain their own web.config files to control individual configuration settings appropriate to the given sub-directory. ASP.Net uses a hierarchal configuration scheme that allows inheritance of configuration settings to minimize the actual web.config entries at a sub-directory.
1.Web.Config file is a configuration files in ASP.Net.
2.By Default only one file you can see in a project but you can add multiple config file.
3.Basically Web.config file is a XML format.
4.You can keep you connection,Session,Keys,Security in a web.config file.
5.You can also handle a error from config file also.