Shutdown the computer immediately (don't power down). Note that in UNIX systems this kind of shutdown means to go to “ single-user mode”. Single-user mode is a mode where only the administrator (root) has access to the computer, this mode is designed for maintenance and is often used for repairs.
For example this would take you to single user mode
Shutdown (-h = halt) the computer immediately. It begins the shutdown procedure, press CTRL-C (break-key) to stop it. After the end of the command you can also leave a message in quotation marks which will be broad-casted to all users, for example:
shutdown -h now "Warning system malfunction, self-destruct imminent"
Shutdown ( -r = reboot) the computer immediately. It begins the reboot procedure, press CTRL-C (break-key) to stop it. After the end of the command you can also leave a message in quotation marks which will be broad-casted to all users, for example:
shutdown -r now "Warning system rebooting, all files will be destroyed"
(key-combination) May be used from a terminal to reboot or shutdown, it depends on your system configuration. Note that this doesn't work from an xterminal. CTRL-ALT-DEL begins the reboot/shutdown immediately, the user does not have to be logged in.
Here is how it looks on a Mandrake/Mandriva Linux system:
Note that the # means a comment (and is not used). If you simply put a # (hash) before the command it would disable it (it would become a comment).
You could also change the command it runs for example if you changed the -r to a -h the computer would turn off instead of rebooting, or you could have it do anything you want. It's up to your creativity to make it do something interesting.