nofollow is an HTML attribute value used to instruct search engines that a hyperlink should not influence the link target's ranking in the search engine's index. Google announced in early 2005 that hyperlinks with rel="nofollow" attribute^[1] would not influence the link target's PageRank. In addition, the Yahoo and MSN search engines also respect this tag.^[2]

How the attribute is being interpreted differs between the search engines. While some take it literally and also not follow the link to the page being linked to, are others still "following" the link to find new web pages for indexing. In the latter case does rel="nofollow" actually tell a search engine "Don't score this link" rather than "Don't follow this link." This differs from the meaning of nofollow as used within a robots meta tag, which does tell a search engine: "Do not follow any of the hyperlinks in the body of this document.".

rel="nofollow" has come to be regarded as a microformat. Microformats reuse existing attributes but extend the standard values for the attribute. "nofollow" is a custom attribute value^.

Interpretation by the individual search engines

While all engines that support the attribute exclude links that use the attribute from their ranking calculation, vary the details about the exact interpretation of the attribute from search engine to search engine. 

• Google takes "nofollow" literally and does not "follow" the link at all. That is supposely their official statement, but experients conducted by SEO's show conflicting results. They show instead that Google does follow the link, but not index the linked to page, unless it was in Google's index already for other reasons (such as other, no-nofollow links that point to the page). ^[5]
• Yahoo! "follows it", but excludes it from their ranking calculation.
• MSN Search supports nofollow regarding not counting the link in their ranking, but it is not proven if or if not MSN follows the link or not.
• Ask.com does not support the attribute at all.