Quoting from Tannenbaum_Operating_Systems_Design :
On top of the operating system is the rest of the system software. Here we find the command interpreter (shell), window systems, compilers, editors, and similar application-independent programs. It is important to realize that these programs are definitely not part of the operating system
I am not getting the difference between these two keywords ,isn't the Windows an Operating system so what's the meaning of this Window Systems which is not the part of Operating System. When we say that windows is an operating system then how can we make a distinction between windows systems and an Operating system ?