How to close GUI and go into pure shell mode
you boot your server in run level 3 or you use CTRL+ALT+F2 go to terminal mode :)
If you switched TTYs with Ctrl+Alt+F1 you can go back to the one running your X with Ctrl+Alt+F7.
TTY 7 is where Ubuntu keeps the graphical interface running.
Its not scary at all, you can use TTYs to do troubleshooting and other necessary configuration, get a bit familiar with it, login, check it out (it works as the terminal inside graphical Ubuntu) and logoff. You can login to several TTYs without messing your system.
When you want to go back to graphical press Ctrl+Alt+F7.