Chris at CSS-Tricks has a huge collection of code snippets for PHP, CSS, jQuery, and, lo and behold, WordPress.
Here’s how you might loop through and display posts in a certain category, for example:
<?php query_posts('cat=5'); ?>
<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>
<?php the_content(); ?>
<?php endwhile; endif; ?>