Here is a description of each fields of the dialog.
A radio button lets you select between a reboot or a shutdown. A shutdown perform the same operations as reboot except that it brings the machine to complete endless stop. The only alternative left after that is to turn off the power or use the reset button to restart it.
On a multi-user system or a server, it may be wise to let people some time to finish their current task before shutting down the computer. If you just want to shutdown or reboot NOW, then enter 0 here. The number is the amount of minutes between the message broadcast and the effective shutdown.
Currently the shutdown feature of Linux can send a message (a small explanation) on terminals. It is expected that this message will be sent to users of other services (samba for one).
You put whatever you want here including nothing. It is simly written to the tty of every user currently logged in.