Excerpt from Wikipeida
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. It is intended to reduce the effectiveness of certain types of spamdexing, thereby improving the quality of search engine results and preventing spamdexing from occurring in the first place.
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.
The concept for the specification of the attribute value
nofollowwas designed by Google’s head of webspam team Matt Cutts and Jason Shellen from Blogger.com in 2005