A hook is a variable where you can store a function or functions (see What Is a Function?) to be called on a particular occasion by an existing program. Emacs provides hooks for the sake of customization. Most often, hooks are set up in the init file (see The Init File), but Lisp programs can set them also. See Standard Hooks, for a list of some standard hook variables.
Most of the hooks in Emacs are normal hooks. These variables contain lists of functions to be called with no arguments. By convention, whenever the hook name ends in ‘-hook’, that tells you it is normal. We try to make all hooks normal, as much as possible, so that you can use them in a uniform way.
Every major mode command is supposed to run a normal hook called the
mode hook as one of the last steps of initialization. This makes
it easy for a user to customize the behavior of the mode, by overriding
the buffer-local variable assignments already made by the mode. Most
minor mode functions also run a mode hook at the end. But hooks are
used in other contexts too. For example, the hook suspend-hook
runs just before Emacs suspends itself (see Suspending Emacs).
If the hook variable’s name does not end with ‘-hook’, that indicates it is probably an abnormal hook. These differ from normal hooks in two ways: they can be called with one or more arguments, and their return values can be used in some way. The hook’s documentation says how the functions are called and how their return values are used. Any functions added to an abnormal hook must follow the hook’s calling convention. By convention, abnormal hook names end in ‘-functions’.
If the name of the variable ends in ‘-predicate’ or ‘-function’ (singular) then its value must be a function, not a list of functions. As with abnormal hooks, the expected arguments and meaning of the return value vary across such single function hooks. The details are explained in each variable’s docstring.
Since hooks (both multi and single function) are variables, their
values can be modified with setq or temporarily with
let. However, it is often useful to add or remove a particular
function from a hook while preserving any other functions it might
have. For multi function hooks, the recommended way of doing this is
with add-hook and remove-hook (see Setting Hooks).
Most normal hook variables are initially void; add-hook knows
how to deal with this. You can add hooks either globally or
buffer-locally with add-hook. For hooks which hold only a
single function, add-hook is not appropriate, but you can use
add-function (see Advising Emacs Lisp Functions) to combine new
functions with the hook. Note that some single function hooks may be
nil which add-function cannot deal with, so you must
check for that before calling add-function.