Nofollow is a kind of HTML attributes that defines hyperlink relationship between the linking page and the linked page.
1. It can be used when you don’t want your links to influence the targeted site’s rankings by search engines. That is, the destination of that hyperlink won’t be given any additional weight or credit in search result rankings.
“From now on, when Google sees the attribute (rel=”nofollow”) on hyperlinks, those links won’t get any credit when we rank websites in our search results.”
Post on Google Blog on 2005-01-18
Not only Google but Yahoo! and MSN Search have agreed to this new hyperlink relationship attribute.
2. This also prevents link spammers from spamming blogs and web forums by posting links in comments and signatures.
WordPress seems to add this attribute by default to the links left in comments.
3. You can use this attribute on some of the links within your website to maximize link values to important content pages.
For example, let’s say that you have a site selling running sneakers. From the homepage, there are two links within the site. One is a link to About Us page and the other is a link to How To Choose Running Sneakers page. Each of these pages gets 50% of the rank value of the homepage in Google, but you don’t really care about how well the About Us page ranks in search engines. In this case, you can add the rel=nofollow attribute to the About Us link. This will give the full score of the homepage rank value to the second page which contains important content for your site.