Wordpress actions for themes and plugins

actions.php

<?php
/* Place this where you need it */
function thing_location(){
  do_action( 'location_of_thing' );
}

/* Make a function to hook */
function add_new_thing() {
  echo 'This is a new thing';
}
/* Hook onto action with function */
add_action( 'location_of_thing', 'add_new_banner', 10 );