example.php

<?php
/**
 * Controlling the class on an article in the WordPress loop
 * Notes: 
 * 
 * <?php post_class(); ?> - this outputs the WordPress generated class for categories
 * You can add classes by adding them inside this function. It will still output the WordPress classes
 *
 */
?>

<?php if ( is_category('example') ) : ?>

	<article id="post-<?php the_ID(); ?>" <?php post_class('example-class'); ?>>

<?php elseif ( is_category('example-2') ) : ?>

	<article id="post-<?php the_ID(); ?>" <?php post_class('example-class-2'); ?>>

<?php else : // if it's not those categories then do this?>

	<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>

<?php endif; ?>