The Server app is only one of the administration tools available for working with OS X Lion Server. The others (Server Admin, Workgroup Manager, Server Monitor, System Image Utility, Podcast Composer, and Xgrid Admin) are all included in the Server Admin Tools 10.7, which is available as a separate download from the Apple web site.
The Server Admin Tools are the standard administration tools that server admins used with previous versions of OS X Server. They provide advanced administration capabilities, letting you set up, configure, and control OS X Lion Server at a much more granular level. While that may seem enticing, the Server app that is included as part of OS X Lion Server provides an interface that is easier to use and can take care of most server needs, even if you have little or no background in administrating or setting up servers. This makes the Server app an ideal place to start if you're new to working with OS X Lion Server; it's also good for experienced server users who just need a quick and simple setup.
If you haven't already downloaded and installed OS X Server, it would probably be a good idea to start with:
Once you have OS X Lion Server installed, let's move on to using the Server app.