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.