Restart Only GUI Session Or Restart Only Operating System?

Yes. There a couple of options.

1) You can enable CTRL+SHIFT+BACKSPACE. This three keys will work like CTRL+ALT+SUPR, but they will only restart the GUI.

2) From the command line (press CTRL+ALT+F1 to get to a console) you can restart the Ubuntu display manager (which lets a user login to her preferred GUI) by entering:

sudo service lightdm restart

3) Finally you can restart just unity. It will reset your GUI (all the windows and the bars), but it will not kill any program. After the restart you can continue working where you left it. Enter:

unity --replace

or if you are restarting from the console (after pressing CTRL+ALT+F1), enter:

DISPLAY=:0 unity --replace

Traditionally this was accomplished in linux by using ctrl + alt + backspace. This has been disabled a few releases ago and it used to be easily enabled (with the dontzap package).

Nowdays I restart X by using the console and type:

sudo killall Xorg

Then the GUI will restart and you'll get back to your login manager. Make sure to save everything first.

To restart your graphical session, log out and log back in.

If the system is frozen, you can use Ctrl-alt-backspace to log you out (kill your applications).

To do that , see http://ubuntuportal.com/how-to-enable-key-sequence-ctrlaltbackspace-to-kill-server-on-ubuntu-11-10/ which gives you a nice graphical walk through.

If you simply want to restart the window manager, without logging out, it depends on the window manager.

For Unity see

How do I restart the window manager?

For gnome you hit Alt + F2 and in the dialog enter r and Enter

If you are running Kubuntu and all your window decorations have disappeared and the task-bar doesn't work, you will probably feel that your GUI session has died. You can restart it while preserving all your running programs if you already have a shell open. Just type:

kwin --replace &