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Help by Command or Variable Name
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   `C-h f' (`describe-function') reads the name of a Lisp function
using the minibuffer, then displays that function's documentation string
in a window.  Since commands are Lisp functions, you can use this to get
the documentation of a command that you know by name.  For example,

     C-h f auto-fill-mode <RET>

displays the documentation of `auto-fill-mode'.  This is the only way
to get the documentation of a command that is not bound to any key (one
which you would normally run using `M-x').

   `C-h f' is also useful for Lisp functions that you are planning to
use in a Lisp program.  For example, if you have just written the
expression `(make-vector len)' and want to check that you are using
`make-vector' properly, type `C-h f make-vector <RET>'.  Because `C-h
f' allows all function names, not just command names, you may find that
some of your favorite abbreviations that work in `M-x' don't work in
`C-h f'.  An abbreviation may be unique among command names yet fail to
be unique when other function names are allowed.

   The default function name for `C-h f' to describe, if you type just
<RET>, is the name of the function called by the innermost Lisp
expression in the buffer around point, _provided_ that is a valid,
defined Lisp function name.  For example, if point is located following
the text `(make-vector (car x)', the innermost list containing point is
the one that starts with `(make-vector', so the default is to describe
the function `make-vector'.

   `C-h f' is often useful just to verify that you have the right
spelling for the function name.  If `C-h f' mentions a name from the
buffer as the default, that name must be defined as a Lisp function.  If
that is all you want to know, just type `C-g' to cancel the `C-h f'
command, then go on editing.

   `C-h w COMMAND <RET>' tells you what keys are bound to COMMAND.  It
displays a list of the keys in the echo area.  If it says the command
is not on any key, you must use `M-x' to run it.  `C-h w' runs the
command `where-is'.

   `C-h v' (`describe-variable') is like `C-h f' but describes Lisp
variables instead of Lisp functions.  Its default is the Lisp symbol
around or before point, but only if that is the name of a known Lisp
variable.  Note: Variables.

   Help buffers describing variables or functions defined in Lisp
normally have hyperlinks to the Lisp definition, if you have the Lisp
source files installed.  If you know Lisp, this provides the ultimate
documentation.  If you don't know Lisp, you should learn it.  If you
are treating Emacs as an object file, then you are just _using_ Emacs.
For real intimacy with Emacs, you must read the source code.