clipped from: * How to stop your Page Rank leak
When to use this:
1. When users can add links by themselves on your site and in your posts so they raise their page rank.
How to use it:
1. Lets say this is your link: <a href="http://www.mysite.com">This is my site</a> put the rel="nofollow" attribute and it will look like this: <a href="http://www.mysite.com" rel="nofollow">This is my site</a>
2. From now on, when Google sees the attribute (rel="nofollow") on hyperlinks, those links won't get any credit when Google ranks websites in their search results.
Why to use it:
1. This isn't a negative vote for the site where the comment was posted; it's just a way to make sure that spammers get no benefit from abusing public areas like blog comments, trackbacks, and referrer lists.