Skin url in magento

<!-- this is inside of .phtml files -->
<img src="<?php echo $this->getSkinUrl('images/breadcrumbs_arrow.gif'); ?>" alt="arrow"/>


<!-- this is in cms block -->
<img class="featured-ratings" src="{{skin url='images/ratings.jpg'}}" alt="" />

Source: http://snipt.net/robertbanh/magento-skin-url/