A no follow link is a link that does not count as a point in the page’s favor, does not boost PageRank, and doesn’t help a page’s placement in the SERPs. No follow links get no love. Theirs is a sad and lonely life.
The no follow link tag has done a lot of good for the online world – most SEO spammers won’t bother posting irrelevant links to blogs or forum posts if they know they won’t get to publish a follow link. WordPress automatically assigns the no follow link attribute to all user-submitted links, and Wikipedia does the same for its reference section.