If you have sections of your page that you dont want google to index, like a comments section that commonly strays off topic, you can use the following HTML comments that the Google bots will listen to:

<p>This part and all previous will be indexed</p>
<!--googleoff:all-->
<p>This part will NOT be indexed</p>
<!--googleon:all-->
<p>This part and rest of the page will be indexed</p>

The all parameter can be changed to any one of the following

• index - will not be indexed
• anchor - any links will not be associated with the target page
• snippet - will not be used to create snippets in search results
• all - index, anchor, and snippet