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 the argument function to get the documentation of a
command that you know by name. For example,
C-h f auto-fill-mode <RET>
displays the documentation for auto-fill-mode. Using C-h f
is the only way to see the documentation of a command that is not bound
to any key, that is, a command you would normally call using M-x.
If the variable describe-function-show-arglist is t,
describe-function shows its arglist if the function is not
an autoload function.
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 make sure 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 function name for C-h f to describe has a default which is
used if you type <RET> leaving the minibuffer empty. The default is
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> (where-is) tells you what
keys are bound to command. It prints a list of the keys in the
echo area. Alternatively, it informs you that a command is not bound to
any keys, which implies that you must use M-x to call the
command.
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, if that is the name of a known Lisp
variable. See Variables.