A server is any computer that respond to requests across a network. That can be a local area network such as all the computers in a company or it could be the entire internet. A web server is a type of server that delivers content through the internet. This web site is sitting on a server some where and when you ask your computer to take you to fluther.com your computer asks Fluther’s web server for the info it need. A windows server is a server that’s running a Windows operating system like Windows Server 2012. It’s possible that a web server is running Windows.

There are such things as a “window manager” and a “display server”, but I don’t know what a “window server” is, unless it’s a server that runs Windows™ as @Lightlyseared suggests.

The word “server” can refer to a program that responds to requests or to a computer that runs those programs. My computer runs several server processes (including a display server) but it’s what you’d normally consider a “server” machine.

A “server” is part of a software architecture (or “design pattern”) called “client – server”. In this paradigm, a program (the client) needing a service contacts another program (the server) which provides it. This communication takes place through something called Inter-Process Communication or “IPC”. IPC can happen over a network or it can happen locally between two processes on the same computer (for example, right now my web browser it telling a display server what its window should look like, and the display server is drawing that window).
The usual sense of a “server” is, as @Lightlyseared states, something that services requests over a network. A display server is an example of one that mostly works locally (although some can also work over a network.)

If you can be more specific about what you mean by a “window server” or provide an example, you might get a better answer.

A web server is a computer program running on a Windows server, a Unix server or some other server operating system. Of course it’s possible to install a web server on a Windows client, but when doing so the machine become a server for other clients.