When to use actions and when to use filters

by

+1 to both of the other answers, for explaining the primary differences between actions and filters.

However, to answer your specific question, about which to use for modifying a query:

  1. Injecting your custom query into the template does, as you guessed, require hooking into an existing action. (Whether or not wp_footer is the correct/ideal action is another matter, and would depend on exactly how you intend to use said query.
  2. Allowing your custom query is probably best accomplished using a custom filter, that other Plugins can hook into.

For example, starting with this code:

function get_page_list() {
    $pages_args = array(
        'post_type' => 'page',
        'posts_per_page' => '1'
    );  
    $pages_query = new WP_Query( apply_filters( 'myfilter', $pages_args ) );


    if( $pages_query->have_posts() ) {
        while ( $pages_query->have_posts() ) : $pages_query->the_post();  
            echo get_the_title();
        endwhile;
        // Reset Post Data
        wp_reset_postdata();
    } 


    else echo 'no results found';                 

}
add_action( 'wp_footer', 'get_page_list', 1);

If you want this code to be extensible, you need to provide a way for others to extend it. I would recommend making your custom query args extensible, via custom filter. e.g. this:

$pages_args = array(
    'post_type' => 'page',
    'posts_per_page' => '1'
);

…could easily become this:

$pages_args = apply_filters( 
    'custom_filter_name', 
    array(
        'post_type' => 'page',
        'posts_per_page' => '1'
    ) 
);

Which would then allow other Plugins to modify your query args. For example, if a Plugin wanted to return two posts, instead of one:

function wpse73840_modify_custom_filter_name( $args ) {
    // Modify posts_per_page
    $args['posts_per_page'] = '2';
    // Return modified args
    return $args;
}
add_filter( 'custom_filter_name', 'wpse73840_modify_custom_filter_name' );

I think this gets to what you’re asking. If not, let me know in the comments.