Note: Google's guidelines on using <link> and <meta> tags to provide alternative machine readable values.
Use the <link> element to provide a URL to identify a resource but not to show a distracting link:
<div itemscope itemtype="http://schema.org/Book">
  <span itemprop="name">The Catcher in the Rye</span>
  <link itemprop="url" href="http://en.wikipedia.org/wiki/The_Catcher_in_the_Rye" />
  by <span itemprop="author">J.D. Salinger</span>
</div>
Use the <meta> tag to provide information that is implicit in the content of the page
 <div itemprop="reviews" 
      itemscope itemtype="http://schema.org/AggregateRating">
    <img src="four-stars.jpg" />
    <meta itemprop="ratingValue" content="4" />
    <meta itemprop="bestRating" content="5" />
    Based on <span itemprop="ratingCount">25</span> user ratings
  </div>