A nofollow link is one which will not be pursued by an Internet search engine’s web crawler. There are certain types of link that Google generally prescribes should be nofollow such as paid advertising links. A link to a site or page is usually seen as a citation backing up the validity of the information on the destination and its potential usefulness given by the provider of the link. If a webmaster wishes to provide the link for users but not provide it as a citation passing ‘link juice’ or credibility in the eyes of the search engines then a nofollow link may be used.
Another common use of nofollow links is where the provision of ‘dofollow’ links would lead to abuse or spam. Many blogs had links that were followed by the search engines placed in comments at the bottom of articles. These proved to be an extremely popular target for spammers who could simply combine their link with an unhelpful message such as ‘great post’ and benefit from the resulting link. By switching to a nofollow comments approach bloggers discourage that kind of activity. Unfortunately with much content spam being automated this attempt to discourage spam is largely a failure.
There is some debate as to the approach taken by search engines to nofollow links and whether the link ‘juice’ is simply lost or passed to the page – or whether indeed search engines do pass some value through nofollow links. It would be interesting to note though that having 100% ‘do follow’ links in your link popularity profile would make it obvious to search engines that a large amount of your links were probably generated for the purposes of ranking. A natural link profile would include users discussing or posting about your site on some nofollow blogs and forums whereas an an artificial campaign that deliberately avoided nofollow links would stand out for precisely that reason. As a result many campaigns will involve building more natural link patterns which include these links.