This category is about Emacs Documentation. See also CategoryHelp.
Emacs comes with a tutorial, which can be accessed using C-h t or the Help menu. It will walk you step-by-step through an editing session in which you will learn the basics of using Emacs as an editor. See EmacsTutorial.
If you wish to extend Emacs, you can do so using EmacsLisp, or Elisp for short. You can learn the basics of simple Emacs Lisp programming by reading the EmacsLispIntro, and a complete reference manual for Emacs Lisp is available in the EmacsLispReference.
See EmacsNewbieHelpReference for finding help in Emacs.
Emacs comes with ReferenceCards. Emacs is known as “the SelfDocumenting” editor, though, so while the refcard can be useful, one should learn to rely on InfoMode for most of the more complicated features of this editor.
If, for some reason, you need access to the Emacs documentation strings without having access to Emacs itself, there’s a site collecting it: Emacs Online Documentation.