The nofollow is a HTML attribute for links. The nofollow attribute supposedly excludes links from search engine ranking calculations. This attribute was first introduced by Google’s Matt Cutts and Blogger.com’s Jason Shellen.
< a href=http://www.your domain.com rel="nofollow">Ancor Text < /a>
It has been found that Google still follows these links, but if the page was not in the index previously they will not index the page, will not show the link as a back link , and will not associate any keyword relevancy from the anchor text of the link.
Yahoo will follow the link and will index the page, show the link as a back link, and will use the keyword relevancy from the anchor text of the link. However they do exclude the link from their ranking calculations.
MSN has not been proven to follow the link. MSN does not count the nofollow links in their ranking calculations.
Sculpting Page Rank
How Can You Tell if a Site Uses Nofollow?
One way of seeing if a site uses the nofollow attribute is to view the web page’s source code. Then you need to comb through the HTML code to find a link. A faster way is to use the SEO for Firefox extension. This Firefox add-on provides a nice option to highlight the nofollow links right on the page.