If this is a single user system, you might want to turn the computer
off when you're done with it.
To do so, you'll have to log into a
special account called root. The root account is the
system adminstrator's account and can access any file on the system.
If you're going to turn the computer off, get the password from the
system adminstrator. (In a single user system, that's you!
Make sure you know the default root password.) Login as
root:
The command shutdown now prepares the system to be reset or turned off. Wait for a message saying it is safe to and then reset or turn off the system. You must go through this procedure, however. You risk losing your work if you don't.
A quick message to the lazy: an alternative to the logout/login
approach is to use the command su. As a normal user, from your
prompt, type su and 
. It should prompt you for the root
password, and then give you root privileges. Now you can shutdown the
system.