When You Should (and Shouldn’t) Use a NoFollow
Link spam has been around for about as long as Google has been using links as a ranking signal. In the early days of Google, black hat SEOs would use blog comments to pass high PR links back to their sites. Then in 2005, Google introduced the “NoFollow” attribute. Any link tagged with a “NoFollow” would be essentially invisible to search engines. The link was still active, but the attribute would make it so the link juice didn’t flow from one site to another.