And yes, it can do all kinds of nice things without additional software (file storage, logons, DHCP, etc.). Software on the server designed to do server-related things (which includes that OS stuff, really), is "server software," not "server.
If you're talking about providing file storage for other computers and centralized logons, then you've just listed three server applications that are delivered with the OS.
I agree that it's "server software" when distinguishing between it and other, non-server, software on the machine.
But if we change our perspective and consider a single application, say the DHCP server, and its interaction with software on other machines (in this case, DHCP clients), then the application is a server.
So I guess that the answer to the question "What is a server?" is dependent on context.
If I'm looking at a network diagram with unlabeled machines and ask which ones are servers, I'm talking about machines.
If I'm talking about a client/server software installation and ask where the server will be installed, "server" refers to the application that is to serve the client applications.
If we're limited to using existing server machines for new installations of server software, I might ask which server we'll be installing the server on.
"That is something up with which I will not put."
- Winston Churchill, regarding the prohibition on ending sentences with prepositions.
Rod Knowlton
IBM Certified Advanced Technical Expert pSeries and AIX 5L
CompTIA Linux+
CompTIA Security+