Nofollow is an attribute that can be added to the HTML hyperlink tag when the quality of an external link is unknown or untrusted.
Because linking to low quality websites can negatively affect rankings, the nofollow attribute acts as a barrier, protecting referring sites from any harm.
Nofollow Rel Attribute Example:
<a href="http://www.example.com/" rel="nofollow">Link text</a>
Major search engines vary slightly in how they interpret the nofollow rel attribute. Contrary to its name, the nofollow rel attribute is "followed" by most search engine spiders, just as all links are, but do not pass PageRank. Google has stated that their spiders do not crawl past nofollowed links; however, this has been disproved. A more appropriate name might have been NoVote.
With Google's new interpretation of
rel=nofollow, the attribute continues to prevent destination URLs from inheriting PageRank; however, it no longer prevents the loss of PageRank associated with outbound links. Instead of conserving the percentage of PageRank blocked from its destination URL, it now vanishes.
Google's new interpretation of
rel=nofollow came about to put an end to PageRank sculpting, the manipulation of the attribute to conserve PageRank, which normally flows through link to link across the Web.
Because a percentage of a web page's PageRank flows to the destination URL of each outgoing link, some have become wary of linking to other sites for fear of losing valuable link equity. In an effort to control and concentrate PageRank on select web pages, webmasters learned to sculpt PageRank using the nofollow attribute.
PageRank sculpting came to an end in 2009, when Google engineer Matt Cutts disclosed that the search engine's interpretation of the nofollow rel attribute had been altered the preceding year.
Some webmasters still try to hoard PageRank in an attempt to conserve link equity and increase their rankings. This effort is misguided, as outbound links to high quality, relevant content is in itself a quality signal that can built a website's trust and rankings.