I think we are all fairly knowledgeable of the nofollow tags that some webmasters put in their links. This kind of tags basically do not pass on Google Pagerank to the sites you link to. It is denoted by the tag rel="nofollow".
Now, a similar tag is usually confused with the nofollow tag. It is the rel="external" tag. Many people are asking if it has anything to do with SEO like nofollow. The answer is a no. Then what is it? Check this article out from Kenneth Dreyer to find out.