Mac OS X Server was an almost identical version of Mac OS X in terms of capability but adds many more features to support Mac, Windows and Linux clients. In Lion, Server can be installed additionally, not requiring a separate Operating System.
Mac OS X Server was an almost identical version of Mac OS X in terms of capability but adds many more features to support Mac, Windows and Linux clients.
In Lion, OS X Server additions can be installed and don't require setting up a separate Operating System.