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Ever performed a desktop search to find the web.config file, only to find that there are multiple web.config files on the same server. A number of web.config files are required to configure an IIS web server and SharePoint?
There will be a separate web.config file for each virtual server in IIS which hosts SharePoint site collections. There is also a web.config file for each of the system directories on a SharePoint site, such as the _vti_bin, _layouts, etc. There is also a web.config file which is used to identify trusted web parts.
To find out which directory is used by various SharePoint applications/websites, view the properties of the SharePoint website in IIS and from the "Home Directory" tab, the value in the "Local path" field will take you to the directory where the web.config file is for the specific instance of SharePoint / Application in IIS.
See Where is SharePoint web.config? for more information about the various web.config files which are required to configure SharePoint & ASP.NET web applications, including inforpation about where to find the web.config file for a SharePoint environment.

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A common change to the web.config file for a SharePoint instance is to enable custom errors, and the stack trace of an error when it occurs. Enabling custom errors will result in a more detailed description being displayed instead of the "An Unexpected Error has Occurred" message or similar. This will help when troubleshooting issues, as it will tell you where the problem is, or at least point you in the right direction.
The article SharePoint Web.Config: How to Show Full Errors explains the various configuration changes which can be made in order to display more information when an error occurs, including enabling custom errors, enabling the call Stack Trace feature, enabling debugging mode, and enabling the ASP.NET tracing feature. Additional information and tips are also provided for when making changes to web.config files relating to some important steps and considerations which should be taken before making and applying changes to a web.config file.