To shut down the system properly, use shutdown. It notifies all users and processes of the impending shutdown, blocks new logins, and brings the system down cleanly. (Just cutting power to the system without cleanly stopping processes and unmounting filesystems could result in the loss or corruption of data.)
To halt the system once the shutdown is complete, use the -h option; to reboot the system after shutdown, use -r instead.
Gnome and KDE users should use the System and K Shut down option.