haltrebootpoweroffshutdownhalt = shutdown -h
reboot = shutdown -r
``halt" set the computer to a mode that enables a user to safely power off the system.
The shutdown command brings the system down in a secure way.
shutdown does its job by signalling the init process, asking it to change the runlevel. Runlevel 0 is used to halt the system, runlevel 6 is used to reboot the system, and runlevel 1 is used to put the system into a state where administrative tasks can be performed (single-user mode). Runlevel 1 is the default, unless the -h or -r options are specified.