Take a look at multiple loops for examples
And also look at the content.php file in the Twenty Fourteen theme
<?php
// The Query
$category_query = new WP_Query( 'category__in=11' );
// The Loop
if ( $category_query->have_posts() ) {
echo '<ul>';
while ( $category_query->have_posts() ) {
$category_query->the_post();
echo '<li>' . get_the_title() . '</li>';
}
echo '</ul>';
} else {
}
/* Restore original Post Data */
wp_reset_postdata();
Don’t forget you can use pre_get_posts to alter an existing loop and exclude categories from any loop.