That depends. But, in general:
Server software is to be run on a server. Windows 2000 Server, Windows NT Server (Is there an XP? I'm not sure, but it sounds wrong.)
Now, the client would run Windows 2000 (a "professional" operating system) or Windows ME (a "home" OS) or Windows XP Home or Windows XP Pro or Win 98 (there was never a server edition of that).
For OS's, there are NT and there are non-NT. The NT is generally the "professional" edition, like Windows XP Pro. Windows 98 was the "home" edition and Windows NT 4 was the "pro" edition.
A mail client is MS Outlook.
A mail server is MS Exchange Server.
A server is a machine in a dark, dusty closet somewhere, usually at work.
A client is the PCs that all the employees use.
You might have your own server at home tho, if you're a techno-geek. That would mean--generally--that you don't keep all your files on your PC, but on the server.
Servers have CPUs just like a regular PC. However, I'm sure it's desirable for them to be more powerful.
Some people have print servers. That would be a PC that's dedicated strictly to processing jobs to a printer--generally at Kinko's and other printhouses, they'll have print servers that run special software.
Most of the keyboarding that would be done at a server is setup kind of stuff. Rarely would someone sit and type up a document on a server.
Am I understanding your question? Giving enough info?