There are two commands for exiting Emacs, one for suspending Emacs and the other for killing Emacs. Suspending means stopping Emacs temporarily and returning control to the shell, allowing you to resume editing later in the same Emacs job, with the same files, same kill ring, same undo history, and so on. This is the usual way to exit. 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 (
suspend-emacs). If used under the X window system,
this command will shrink the X window containing the Emacs frame to an
icon. Clicking on the icon will resume that Emacs process
again. See Exiting Emacs in SXEmacs User’s Manual.
Kill Emacs (
save-buffers-kill-emacs). You can also select
Exit Emacs option from the File menu to kill that Emacs
process. If you haven’t saved the file, Emacs will ask you if you wish
to save the file before killing that process.