Filters and Actions (Hooks)

Hook into an action with add_action(). Example:

function group_is_gone_message( $group_id ) {
 echo "Group {$group_id} is gone!";
}
add_action( 'term_group_deleted', 'group_is_gone_message' );

Hook into a filter with add_filter(). The function should return the modified first argument. Example:

function wrap_as_gift( $html ) {
 return "<div class='wrapping_paper'>" . $html . "</div>";
}
add_filter( 'tag_groups_post_terms', 'wrap_as_gift' );