If you want to shut down the computer, usually "poweroff" or "shutdown -h now" would do.
If you want to exit from the graphical interface, you typically just need to shut down the X server (the graphical session generally runs through a display manager session or your startx command)
no
The GUI uses unnecessary resources.
Yes and no. Yes, Linux has a GUI. Dozens, in fact, all running on the X Windowing System. No, Linux is not BASED on a GUI. Only Windows really has the concept of an operating system "based" on a GUI. To Linux, the X Windowing System and whatever you run on top of it is just another application.
Not sure what your question is asking, other than the GUI is displayed by the desktop manager.
Yes
X Windows
gnome and kde
The Linux kernel itself does not include a GUI. However, most Linux distributions have one. Yes, Linux have a GUI.Today many kind of vendor available in Linux Like Redhat fedora,Ubuntu,Suse, Mandriva and many more. Those Linux version provide Cmd and GUI mode for work.
x windows
There are many differences between the Graphical User Interfaces of Windows and Linux. There are similarities as well. To be exact, Linux, itself has no GUI per se. The GUI(s) that you can use, and there are literally hundreds to choose from, are separate programs running under Linux. [speaking simply here]Linux itself is the core (kernel) of the Operating System, whereas the GUI, the file managers, web browsers, chat programs etc, are all applications that give the OS "features". With Windows you only have the included GUI, that is unless you install a modification such as WindowBlinds(TM). Linux, and many other POSIX compliant OS's can use many GUI, here is a brief listing and some links to GUI sites. KDE - The K Desktop Environment: http://kde.org/Gnome (Pronounced with a hard "Guh" as in Guh-Nome: http://gnome.org/Enlightenment: http://www.enlightenment.org/blackbox: http://sourceforge.net/projects/blackboxwmAfterStep: http://freshmeat.net/projects/afterstep/ and many more...
Many Linux users find the command line quicker than using the Graphic User Interface (GUI).
The ability to run a GUI is commonly called X11 with different implementations. The most popular in the Linux world is called X.Org Server. There are several but the most popular it seems are Gnome, KDE, and Xfce.