The start of Windows Server can be discussed, one can argue that it started with Windows NT 3.1 back in 1993 or you can argue it really started with Window Server 2000. If you start with Windows NT 3.1 then you can follow the line through NT 3.5, NT 3.51, NT 4 and on to Windows 2000. Alternatively it was really Windows Server 2003 that gained the stronger server branding.
More information on Windows Server can be found on Wikipedia at Windows Server which I have to admit is a great resource.
Sometimes it is handy to know the latest service pack for a given Operating System, so here is a short and handy list:
There is information under Microsoft Windows that will also apply to the Windows Server range of products.