Here's a basic skeleton for a loop called using the WP_Query class. You could use this to display a secondary loop on the homepage, for example, a block featuring specific categories.

using-wpquery-class.php

<?php
$args = array(
    // arguments for your query
);
// the query
$query = new WP_Query( $args );

// The Loop
if ( $query->have_posts() ) :  while ( $query->have_posts() ) : $query->the_post() ;
	// contents of the Loop go here

endwhile; endif;
/* Restore original Post Data */
wp_reset_postdata();
?>