The nofollow attribute value, also called nofollow tag, is used to specifically instruct search engines to not follow one or multiple links on a site. This will cause them to not crawl the linked pages.
The nofollow attribute value can be used both for a specific link, with the link attribute rel=”nofollow”, as well as for an entire document through the meta robots attribute.
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Why does the nofollow attribute value exist?
nofollow is modelled after a concept for spam prevention from Matt Cutts (Google Software Engineer) and Jason Shellen (Blogger Program Manager). Google first introduced the “nofollow” attribute as a standard in 2005 to give webmasters a tool for identifying which links on a page should not be followed by searchengines.
Later, Google also added some guidelines for using the nofollow-attribute for specific links.
You add the “nofollow” value to a single link like this:
<a href="http://www.domain.com" rel="nofollow">I'm an external link</a>
The consequences: search engines will not follow this links and therefore also refrain from crawling the target page. In addition to that, Google will not transfer any PageRank and anchor text from the link-giving page to the target page. This link will therefore not be considered for the calculation of the link popularity anymore.
Please also note Google’s clarification:
However, the target pages may still appear in our index if other sites link to them without using nofollow, or if the URLs are submitted to Google in a Sitemap.Search Console Help
Page-specific use in the meta-robots-attribute
You can use the appropriate nofollow-value in the meta-robots-attribute of a page to keep all links on that page from being followed by searchengines:
<meta name="robots" content="nofollow">
The consequences: search engines will not follow any links, internal or external, on the entire page. Additionally, they will neither use the PageRank nor the anchor text for the calculation of the link popularity.