The Loop
<?php
if ( have_posts() ) {
while ( have_posts() ) {
the_post();
<!-- Test if the current post is in category 3. -->
<!-- If it is, the div box is given the CSS class "post-cat-three". -->
<!-- Otherwise, the div box is given the CSS class "post". -->
<?php if ( in_category( '3' ) ) : ?>
<div class="post-cat-three">
<?php else : ?>
<div class="post">
<?php endif; ?>
<!-- Display the Title as a link to the Post's permalink. -->
<h2><a href="<?php the_permalink(); ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></h2>
<small><?php the_time('F jS, Y'); ?> by <?php the_author_posts_link(); ?></small>
<div class="entry">
<?php the_content(); ?>
</div>
<p class="postmetadata"><?php _e( 'Posted in' ); ?> <?php the_category( ', ' ); ?></p>
</div> <!-- closes the first div box -->
} // end while
} // end if
?>
See More at -
Codex: The Loop