wp_footer
wp_footer is triggered in the "footer" section of the user's template. This location may vary from theme to theme.
This hook is theme-dependant which means that it is up to the author of each WordPress theme to include it. It may not be available on all themes, so you should take this into account when using it.
This hook is an action which means that it primarily acts as an event trigger, instead of a content filter. This is a semantic difference, but it will help you to remember what this hook does if you use it like this: add_action('wp_footer', 'your_function');