Nofollow

nofollow is a value that can be assigned to the rel attribute of an HTML a element to instruct some search engines that the hyperlink should not influence the ranking of the link's target in the search engine's index. It is intended to reduce the effectiveness of certain types of internet advertising because their search algorithm depends heavily on the number of links to a website when determining which websites should be listed in what order in their search results for any given term.

The nofollow value was originally suggested to stop comment spam in blogs. Believing that comment spam affected the entire blogging community, in early 2005 Google's Matt Cutts and Blogger's Jason Shellen proposed the value to address the problem.

The specification for nofollow is copyrighted 2005–07 by the authors and subject to a royalty-free patent policy, e.g. per the W3C Patent Policy 20040205, and IETF RFC 3667 & RFC 3668.

<a href="http://www.example.com/" rel="nofollow">Link text</a>

Google announced in early 2005 that hyperlinks with rel="nofollow" would not influence the link target's PageRank. In addition, the Yahoo and Bing search engines also respect this attribute value.

On June 15, 2009, Google software engineer Matt Cutts announced on his blog that GoogleBot changed the way it treats nofollowed links, in order to prevent webmasters from using nofollow for PageRank sculpting. Prior to this, webmasters would place nofollow tags on some of their links in order to maximize the PageRank of the other pages. As a result of this change, the usage of nofollow leads to evaporation of pagerank of outgoing normal links as they started counting total links while calculating page rank. The new system divides page rank by total number of out going links irrespective of nofollow or follow links, but passes the page rank only through follow or normal links. Cutts explained that if a page has 5 normal links and 5 nofollow out going links, the page rank will be divided by 10 links and one share is passed by 5 normal links.

...
Wikipedia