Nofollow links were introduced by Google to mark links that should not be used for the ranking algorithm. Its main purpose was to stop comment spam in blogs. The idea was that people would stop spamming blogs and other pages with links if these links didn't influence the rankings anymore.

To mark a link as a nofollow link, it gets rel="nofollow" as an attribute in the HTML code. Links with that attribute will be ignored by ranking algorithms that use links to determine the position of a web page.