What is hooks and types of hooks in wordpress?Where it used?

Hooks are provided by WordPress to allow your plugin to ‘hook into’ WordPress; that is, to call functions in your plugin at specific times, and thereby set your plugin in motion. There are two types of hooks used in WordPress are Actions and Filters.

1)Actions Run During a Typical Request.for example A developer may want to add code to the footer of a Theme. This could be accomplished by writing new function, then Hooking it to the wp_footer Action.
has_action()
add_action()
do_action()
do_action_ref_array()
did_action()
remove_action()
remove_all_actions()

2)Filters are the hooks that WordPress launches to modify text of various types before adding it to the database or sending it to the browser screen.
has_filter()
add_filter()
apply_filters()
apply_filters_ref_array()
current_filter()
remove_filter()
remove_all_filters()