Killing Emacs means ending the execution of the Emacs process.
If you started Emacs from a terminal, the parent process normally
resumes control. The low-level primitive for killing Emacs is
kill-emacs.
This command calls the hook kill-emacs-hook, then exits the
Emacs process and kills it.
If exit-data is an integer, that is used as the exit status of the Emacs process. (This is useful primarily in batch operation; see Batch Mode.)
If exit-data is a string, its contents are stuffed into the terminal input buffer so that the shell (or whatever program next reads input) can read them.
If exit-data is neither an integer nor a string, or is omitted, that means to use the (system-specific) exit status which indicates successful program termination.
If restart is non-nil, instead of just exiting at the
end, start a new Emacs process, using the same command line arguments
as the currently running Emacs process.
The kill-emacs function is normally called via the
higher-level command C-x C-c
(save-buffers-kill-terminal). See Exiting in The GNU
Emacs Manual. It is also called automatically if Emacs receives a
SIGTERM or SIGHUP operating system signal (e.g., when the
controlling terminal is disconnected), or if it receives a
SIGINT signal while running in batch mode (see Batch Mode).
This normal hook is run by kill-emacs, before it kills Emacs.
Because kill-emacs can be called in situations where user
interaction is impossible (e.g., when the terminal is disconnected),
functions on this hook should not attempt to interact with the user.
If you want to interact with the user when Emacs is shutting down, use
kill-emacs-query-functions, described below.
When Emacs is killed, all the information in the Emacs process,
aside from files that have been saved, is lost. Because killing Emacs
inadvertently can lose a lot of work, the
save-buffers-kill-terminal command queries for confirmation if
you have buffers that need saving or subprocesses that are running.
It also runs the abnormal hook kill-emacs-query-functions:
When save-buffers-kill-terminal is killing Emacs, it calls the
functions in this hook, after asking the standard questions and before
calling kill-emacs. The functions are called in order of
appearance, with no arguments. Each function can ask for additional
confirmation from the user. If any of them returns nil,
save-buffers-kill-emacs does not kill Emacs, and does not run
the remaining functions in this hook. Calling kill-emacs
directly does not run this hook.
This command does the same as save-buffers-kill-emacs, but
instead of just killing the current Emacs process at the end, it’ll
restart a new Emacs process, using the same command line arguments as
the currently running Emacs process.