A NoFollow Link is a hyperlink to your website that does not deliver search engine ranking benefits.
To create a NoFollow Link, the rel=nofollow attribute must be added to the hyperlink in the HTML, like this:
<a href="http://www.example.com/" rel="nofollow">Link text</a>
Most news websites and many high traffic blogs only use NoFollow Links, so as not to favor any linked domains.
All ads should have a NoFollow Link, per Google Webmaster guidelines.
A link that does pass domain authority is called a Follow Link.