Emacs Lisp is what makes Emacs ExtensibleSoftware. Customizations and extensions can be made quickly and portably. Some of the ways that Emacs Lisp is used to extend Emacs include hooking custom Emacs Lisp functions into events, binding functions to key combinations, creating new editing modes and even writing whole applications for the WorldWideWeb or other Internet clients.
Lisp has been rumored to stand for Lots of Insignificant Silly Parentheses, but it really means LISt Processor.
EmacsLispReference is the official documentation.
For help on using other people’s Emacs Lisp, see InstallingPackages.
((1 . 2) . (3 . 4))
`(foo ,bar baz)
If you are wondering about all the quoting syntaxes like
?a 'b #c `(,d e) :f (g . h) etc., then see EmacsSymbolNotation.