Question about query_posts

Hello!

I would like to exclude category from the loop and I have some question:

<?php if ( is_home() ) {
			query_posts($query_string . '&cat=-124');
			}
		?>
        <?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>
        	<?php get_template_part( 'content', get_post_format() ); ?>
        <?php endwhile; else: ?>
            <p><?php _e('empty'); ?></p>
        <?php endif; ?>

A guy told me I have to use global $post, posts instead of $query_string, whats the difference? I read in the codex I have to run wp_reset_query(); after i “finished”? Why? And where have to put that code? (wp_reset_query();)

Thank you very much!