Within Emacs, you can type C-h f to get the documentation for a function, C-h v for a variable.
For more information, the Emacs Lisp Reference Manual is available in Info format (see Emacs Lisp).
You can also order a hardcopy of the manual from the FSF, for details see the FSF on-line store. (This manual is not always in print.)
An HTML version of the Emacs Lisp Reference Manual is available at