Building back-links to your site is an important strategy for any internet marketer. However, not all links are equally valuable, and some may even be worthless. Many blogs, forums and article directories add an attribute called nofollow to their outbound links to discourage people from spamming the site with irrelevant links.
A normal dofollow link on a web site will look something like this:
<a href="http://www.link.com">Link</a>
To make an individual link nofollow, the value is added to the rel attribute in the html link tag:
<a href="http://www.link.com" rel="nofollow">Link</a>
The nofollow attribute was developed in 2005 by Google to help combat link spam in the comment section of blogs. It was designed so that webmasters could attach it to all unwanted links that are unrelated to their site, so that Google could improve the quality of their search engine results.