To shut down a running Debian GNU/Linux system, you must not reboot with the reset switch on the front or back of your computer, or just turn off the computer. Debian GNU/Linux should be shut down in a controlled manner, otherwise files might get lost and/or disk damage might occur. If you run a desktop environment, there is usually an option to “log out” available from the application menu that allows you to shutdown (or reboot) the system.
Alternatively you can press the key combination Ctrl+Alt+Del or Control+Shift+Power on Macintosh systems. A last option is to log in as root and type one of the commands poweroff, halt or shutdown -h now if either of the key combinations do not work or you prefer to type commands; use reboot to reboot the system.