This chapter talks about various topics relevant to adapting the behavior of Emacs in minor ways. See [The Emacs Lisp Reference Manual] for how to make more far-reaching changes.
All kinds of customization affect only the particular Emacs session that you do them in. They are completely lost when you kill the Emacs session, and have no effect on other Emacs sessions you may run at the same time or later. The only way an Emacs session can affect anything outside of it is by writing a file; in particular, the only way to make a customization "permanent" is to put something in your .emacs file or other appropriate file to do the customization in each session. Section 32.7.