Hi Vinay,
IIS Custom errors are Error messages for HTTP Errors some of them are as below
200 - OK
400 - Bad request
401 - Access denied
401.1 - Logon failed
403.1 - Execute access forbidden
404.0 - (None) – File or directory not found
Web.Config Custom error messages are of three types
="1.0"="utf-8"
<configuration>
<system.web>
<customErrors defaultRedirect="url"
mode="On|Off|RemoteOnly">
<error statusCode="500"
redirect="InternalError.htm"/>
</customErrors>
</system.web>
</configuration>
If Mode is OFF then this specifies that custom errors are disabled. This allows display of detailed ASP.net errors
If Mode is ON then this specifies that custom errors are enabled. If no
defaultRedirect is specified, users see a generic ASP.NET error
If Mode is Remote then this specifies that custom errors are shown only to remote clients and ASP.NET errors are shown to the local host. This is the default.
defaultRedirect :Specifies the default URL to direct a browser to if an error occurs. When defaultRedirect is not specified, a generic error is displayed instead