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A hook is a variable where you can store a function or functions 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 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).
The recommended way to add a hook function to a hook is by calling
add-hook (see Setting Hooks). The hook functions may be any
of the valid kinds of functions that funcall accepts (see What Is a Function). 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.
If the hook variable's name does not end with ‘-hook’, that
indicates it is probably an abnormal hook. That means the hook
functions are called with arguments, or their return values are used
in some way. The hook's documentation says how the functions are
called. You can use add-hook to add a function to an abnormal
hook, but you must write the function to follow the hook's calling
convention. By convention, abnormal hook names end in ‘-functions’.
If the variable's name ends in ‘-function’, then its value is
just a single function, not a list of functions. add-hook cannot be
used to modify such a single function hook, and you have to use
add-function instead (see Advising Functions).