SEO/SEM Base de connaissances
Are nofollow links important? under Linkbuilding
Nofollow is an attribute of html link, or anchor element, used to prevent the website’s rank from being affected by users’ posted comments.
Using a nofollow attribute in a comments section, or whenever it is necessary on a web page, will help you protect yourself against spammers.
Search engines will not consider such links as ones which are being taken into consideration, while calculating your rank and will make spam ineffective, for the terms of SEO.