What is rel="nofollow"?
It tells crawlers to map out links with rel="nofollow" out of your link graph and thereby stopping the distribution of page rank to it.
What does Matt Cutts say about rel="nofollow"?
Why and When to use rel="nofollow"?
Its clear that if its your internal links, you'd want page rank to flow all over. But with external links which you just happen to mention and don't really want to spread Google juice or page rank, then you can use rel="nofollow".
What does Matt Cutts say about when to use rel="nofollow"?
How to Use rel="nofollow"?
Just add rel="nofollow" (no space in between)
<a href="http://seosemblog.byethost14.com" rel="nofollow">Seo Blog</a>
In summary, rel="nofollow" can increase or decrease your page rank depending on how you use it. Remember to be mindful so you don't exert too much effort in it and end up not having good page rank results.
