nofollow is a value that can be assigned to the rel attribute of an HTML a element to instruct some search engines that a hyperlink should not influence the link target's ranking in the search engine's index. It is intended to reduce the effectiveness of certain types of search engine spam, thereby improving the quality of search engine results and preventing spamdexing from occurring.
Read more about Nofollow: Concept and Specification, Introduction and Support, Interpretation By The Individual Search Engines, Use By Weblog Software, Use On Other Websites, Criticism