Server Assistant is the first program that is run after an install of Mac OS X Server

Mac OS X Server

Mac OS X Server is a Unix server operating system from Apple Inc. The server edition of Mac OS X is architecturally identical to its desktop counterpart, except that it includes work group management and administration software tools...

. It can be run again to execute further configuration on a remote or local server. It is also capable of executing remote installation of software onto the server as well.

Server Assistant is a software wizard

Wizard (software)

A software wizard or setup assistant is a user interface type that presents a user with a sequence of dialog boxes that lead the user through a series of well-defined steps. Tasks that are complex, infrequently performed, or unfamiliar may be easier to perform using a wizard...

 that guides the administrator through setting up functions of Mac OS X Server.

It is accessible from Finder > Applications > Server > Server Assistant