The command line program to shut down your computer is called “shutdown”, which should come as no surprise.
Running “shutdown” in a command prompt with no parameters lists all the available parameters.
Usage: shutdown [/i | /l | /s | /r | /g | /a | /p | /h | /e | /o] [/hybrid] [/soft] [/fw] [/f] [/m \\computer][/t xxx][/d [p|u:]xx:yy [/c "comment"]] No args Display help. This is the same as typing /?. /? Display help. This is the same as not typing any options. /i Display the graphical user interface (GUI). This must be the first option. /l Log off. This cannot be used with /m or /d options. /s Shutdown the computer. /r Full shutdown and restart the computer. /g Full shutdown and restart the computer. After the system is rebooted, restart any registered applications. /a Abort a system shutdown. This can only be used during the time-out period. Combine with /fw to clear any pending boots to firmware. /p Turn off the local computer with no time-out or warning. Can be used with /d and /f options. /h Hibernate the local computer. Can be used with the /f option. /hybrid Performs a shutdown of the computer and prepares it for fast startup. Must be used with /s option. /fw Combine with a shutdown option to cause the next boot to go to the firmware user interface. /e Document the reason for an unexpected shutdown of a computer. /o Go to the advanced boot options menu and restart the computer. Must be used with /r option. /m \\computer Specify the target computer. /t xxx Set the time-out period before shutdown to xxx seconds. The valid range is 0-315360000 (10 years), with a default of 30. If the timeout period is greater than 0, the /f parameter is implied. /c "comment" Comment on the reason for the restart or shutdown. Maximum of 512 characters allowed. /f Force running applications to close without forewarning users. The /f parameter is implied when a value greater than 0 is specified for the /t parameter. /d [p|u:]xx:yy Provide the reason for the restart or shutdown. p indicates that the restart or shutdown is planned. u indicates that the reason is user defined. If neither p nor u is specified the restart or shutdown is unplanned. xx is the major reason number (positive integer less than 256). yy is the minor reason number (positive integer less than 65536).
As you can see, it has many options. The most common usages are:
which shuts down the computer completely, closing (or force-closing) any running programs in order to do so; and
which reboots the computer, returning you to the Windows log-in screen.
The most common use of the shutdown command might be in batch files, or similar scripts which perform some function and then need to automatically reboot the machine when done. For many years, I ran a nightly script that included a reboot using shutdown, simply because my machine seemed more stable the next day. (Fortunately, those instabilities have passed, and I no longer feel the need to reboot every night.)