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There are two commands for exiting Emacs because there are three kinds of exiting: suspending Emacs, Iconifying Emacs, and killing Emacs.
Suspending means stopping Emacs temporarily and returning control to its parent process (usually a shell), allowing you to resume editing later in the same Emacs job, with the same buffers, same kill ring, same undo history, and so on. This is the usual way to exit Emacs when running on a text terminal.
Iconifying means replacing the Emacs frame with a small box somewhere on the screen. This is the usual way to exit Emacs when you're using a graphics terminal.
Killing Emacs means destroying the Emacs job. You can run Emacs again later, but you will get a fresh Emacs; there is no way to resume the same editing session after it has been killed.
suspend-emacs) or iconify a frame
(iconify-or-deiconify-frame).
save-buffers-kill-emacs).
To suspend or iconify Emacs, type C-z (suspend-emacs).
On text terminals, this suspends Emacs. On graphics terminals,
it iconifies the Emacs frame.
Suspending Emacs takes you back to the shell from which you invoked Emacs. You can resume Emacs with the shell command %emacs in most common shells. On systems that don't support suspending programs, C-z starts an inferior shell that communicates directly with the terminal. Emacs waits until you exit the subshell. (The way to do that is probably with C-d or exit, but it depends on which shell you use.) The only way on these systems to get back to the shell from which Emacs was run (to log out, for example) is to kill Emacs.
Suspending can fail if you run Emacs under a shell that doesn't
support suspending programs, even if the system itself does support
it. In such a case, you can set the variable cannot-suspend to
a non-nil value to force C-z to start an inferior shell.
(One might also describe Emacs's parent shell as “inferior” for
failing to support job control properly, but that is a matter of
taste.)
On graphics terminals, C-z has a different meaning: it runs
the command iconify-or-deiconify-frame, which temporarily
iconifies (or “minimizes”) the selected Emacs frame
(see Frames). Then you can use the window manager to get back to
a shell window.
To exit and kill Emacs, type C-x C-c
(save-buffers-kill-emacs). A two-character key is used for
this to make it harder to type by accident. This command first offers
to save any modified file-visiting buffers. If you do not save them
all, it asks for reconfirmation with yes before killing Emacs,
since any changes not saved will be lost forever. Also, if any
subprocesses are still running, C-x C-c asks for confirmation
about them, since killing Emacs will also kill the subprocesses.
If the value of the variable confirm-kill-emacs is
non-nil, C-x C-c assumes that its value is a predicate
function, and calls that function. If the result is non-nil, the
session is killed, otherwise Emacs continues to run. One convenient
function to use as the value of confirm-kill-emacs is the
function yes-or-no-p. The default value of
confirm-kill-emacs is nil.
There is no way to resume an Emacs session once you have killed it. You can, however, arrange for Emacs to record certain session information when you kill it, such as which files are visited, so that the next time you start Emacs it will try to visit the same files and so on. See Saving Emacs Sessions.
The operating system usually listens for certain special characters whose meaning is to kill or suspend the program you are running. This operating system feature is turned off while you are in Emacs. The meanings of C-z and C-x C-c as keys in Emacs were inspired by the use of C-z and C-c on several operating systems as the characters for stopping or killing a program, but that is their only relationship with the operating system. You can customize these keys to run any commands of your choice (see Keymaps).