if there is a user currently logged in

<?php if ( is_user_logged_in() ) { ?>
     <!-- text that logged in users will see -->
  <?php } else {   ?>
<!-- here is a paragraph that is shown to anyone not logged in -->
<p>By <a href="<?php bloginfo('url'); ?>/wp-register.php">registering</a>,
 you can save your favorite posts for future reference.</p>
<?php } ?>  

if there is a user currently logged in