Server Core is an install-time option for Server 2008. Microsoft does not support upgrading (downgrading?) to Server Core from the "full" version of Server 2008, nor can you perform an in-place migration from Server Core to the full version. A clean install is your only choice. (However, you should be able to upgrade from one Beta version of Server Core to the next without having to perform a from-scratch reinstall.)

Here's a sampling of the tools available at the command line include the following (most of the EXE's reside in C:\Windows\System32):
• BCDEDIT to customize the boot configuration
• CACLS to display and modify file ACL's
• CMD to spawn a new instance of the command interpreter
• DISKPART to manage disk partitions
• DRIVERQUERY to see what device drivers are installed
• FTYPE to manage file suffix associations that live in HKEY_CLASSES_ROOT
• GPRESULT to evaluate applied Group Policy settings
• NETSH to configure network settings
• SC to display, configure, start, and stop services (for example, SC QUERY shows all services and their state)
• SCHTASKS to manage scheduled tasks
• START to open a new window to run a particular program
• SYSTEMINFO to view a variety of specs for the particular system
• REGEDIT to manage the Registry (works as usual, in GUI mode)

You'll note that some of the tools, such as REGEDIT, do bring up the graphical version of the utility, even though the Explorer shell is certainly not present. Other examples include Notepad and Task Manager. (Just don't expect to use the help system.)

Nearly all of the command line tools provide some documentation when you invoke them with the /? parameter.

The command line is also available remotely via Terminal Services in its "remote administration" mode. To turn this on, use the included SCREGEDIT.WSF script in the C:\Windows\System32 folder.

You are allowed to open multiple instances of the command prompt in Server Core. And, if you do something silly like close the last (or only) command prompt window, you may do what I did and stare foolishly at the empty blue background until you remember that Task Manager works in Server Core, and you can use the New Task button to run an instance of CMD.EXE again.