difference between windows and red hat linux
 

most of the people say that Red Hat Linux is more secure than windows on the server end.
in what security reason Linux is robust than windows

That's sort of vague. Linux and Windows vast differently from one another. One is a clone of the *nix OS and Windows well, is Windows.

The main reason though Linux is more secure is the way user accounts and permissions are setup. A virus in Windows doesn't have the same affect that it does in Linux as an example.

Every computer is vulnerable though in a sense of security, it really boils down to the administrator of the machine making it more secure.