Windows Server is basically just another Windows operating system like XP or Vista. The big difference is the software bundled with it. Windows Server 2003 and 2008 have several "server" programs such as IIS, Active Directory, DNS, WINS, Terminal Services, Print Server, etc., etc., which desktop operating systems don't have.
That's all that is really different. Windows Server 2008 even has the same kernel that Windows 7 does.