When the shutdown command is initiated, a warning followed by a final shutdown message is broadcast to all users currently logged onto the system and all systems that are mounting resources from the affected system.
This is why the shutdown command is recommended over the init command when used on a server. When using either command, you might want to give users more notice by sending a mail message about any scheduled system shutdown.
Use the who(1) command to determine which users on the system need to be notified. This command is also useful for determining a system's current run level, which is described on "How to Determine a System's Run Level".