Do not just turn off the computer! You risk losing valuable data!
Unlike most versions of DOS , it's a bad thing to just hit the power switch when you're done using the computer. It is also bad to reboot the machine (with the reset button) without first taking proper precautions. Linux, in order to improve performance, caches the disk. This means it temporarily stores part of the permanent storage in RAM. The idea of what thinks the disk should be and what the disk actually contains is syncronized every 30 seconds. In order to turn off or reboot the computer, you'll have to go through a procedure telling it to stop caching disk information.
If you're done with the computer, but are logged in (you've entered a username and password), first you must logout. To do so, enter the command logout. All commands are sent by pressing the key marked ``Enter'' or ``Return''. Until you hit enter, nothing will happen, and you can delete what you've done and start over.
Now another user can login.