When you are finally done using your computer, don't forget to tell the system you are leaving, that is remember to log out in a proper manner.
Logging out can be carried out in many ways: from the main menu, or from the menu that appears when you right-click on the desktop.
Whichever method you use, the screen will shade and a little box will pop up with options. If you click on OK you will quit the current session and, after all of your windows and the desktop itself have closed, you will be returned to the login screen.
However there are two other options available in the confirmation window: you can choose to shutdown the system (power off the computer), as well as reboot your system. Again, just click on the OK button after you've selected the desired option.
This is the correct and safe way to shut down or reboot your system. You should never do it by pushing your computer's power button because this can lead to serious problems such as file-system corruption or the loss of data.