Links can have lots of attributes.

Links can have lots of attributes. The engines ignore nearly all of them, with the important exception of the rel="nofollow" attribute. In the example above, adding the rel="nofollow" attribute to the link tag tells the search engines that the site owners do not want this link to be interpreted as an endorsement of the target page.

Nofollow, taken literally, instructs search engines to not follow a link (although some do). The nofollow tag came about as a method to help stop automated blog comment, guest book, and link injection spam (read more about the launch here), but has morphed over time into a way of telling the engines to discount any link value that would ordinarily be passed. Links tagged with nofollow are interpreted slightly differently by each of the engines, but it is clear they do not pass as much weight as normal links.