FYI - The Mac OS X User Interface itself is an application that runs on top of the underlying UNIX system. You can interact with the UNIX system through the text terminal shell environment through /Applications/Utilities/Terminal.app or in an xterm under /Applications/Utilities/X11.app an optional install on the OS X DVD.
The Welcome to Darwin message appears when you enter that UNIX environment, which is basically the contents of a file /etc/motd or the message of the day, administrators can modify this to tell people important news or whatever when they log in. So when you had booted into the UNIX system something was preventing the Mac OS X user login application from starting up, but the UNIX system started up and printed the /etc/motd to the screen. Normally you don't see this text terminal window because the Mac OS X should start automatically in the boot process.
What's useful about all that and why would you care? Well, there are many freely available software applications for GNU Linux systems that can also run on the Darwin environment of Mac OS X, a package manager is a utility that helps you install these, check out the fink package manager if you are interested,
http://www.finkproject.org/
to compile packages from source code you need to install the optional Developer Tools, X11 and X11 SDK from the Mac OS X install DVD. Some GNU linux software is also available as a binary download ready to run under Mac OS X, with no package manager or command-line shell terminal required.
http://www.openoffice.org/
http://www.macgimp.org/
http://www.gimpshop.com/
Most will run in 10.3 or 10.4, not sure about updates for 10.5 yet...