edit NoFollow links
The NoFollow attribute was introduced so that website owners could selectively tell search engine spiders to ignore specific links when it comes to determining the search engine ranking or PageRank of the target web page.
This is what the HTML code for a NoFollow links looks like:
<a href="http://example.com" rel="nofollow">anchor text</a>
NoFollow links are now commonplace in certain types of websites. For example, most blogs are configured so that all links in the comments section are NoFollow to deter people from filling up comments with spammy links. Also, many popular social networks - such as Facebook and Twitter - set all links in people's posts as NoFollow by default. Again, this is to prevent spam.
On the Web, a link to a website is essentially a vote for that site. You may not want search engines to think you're endorsing or connected with some of the websites you link to. If that's the case, marking the link with the NoFollow attribute is one way to show search engines you're not recommending that they add the site to their indexes.
For more detailed guidance on deciding which links to mark NoFollow, read No Follow Links Don't Help PageRank or SEO.
Here on AboutUs.org - the world's largest editable website directory - we link to every listed website. These links are NoFollow by default because we know that every site on the Web isn't great, nor necessarily worthy of our endorsement. We choose to follow the links on a Website Profile once we have reviewed the site and the page about it. See the AboutUs DoFollow links policy for more details.