Seperating Categories

I have this code using the WordPress loop, but of course it pulls all categories from the database. How can I customize the loop so it only selects certain cateogories?

<?php if (have_posts()) : ?>
	<?php while (have_posts()) : the_post(); ?>
	<div class="post_container">
	<div class="title"><h2><a href="<?php the_permalink() ?>"><?php the_title(); ?></a></h2></div>
	<div class="thumb"><img src="" alt="" /></div>
	<div class="desc"><p><?php the_content('Read the rest of this entry &raquo;'); ?></p></div>
	<?php endwhile; ?>
	<?php endif; ?>