Alright, there are two main values that are used in the rel attribute for the anchor tag: nofollow and external.
no follow
nofollow will tell search engines that this link is not trustworthy. So what means is that Google will not give any credit to this link for your site. It’s not going to pass any PR to your site but it might still bring you some traffic. Even if it shows as a backlink to your site, it’s of no value, remember that.
Here’s the URL that talks about it from Google Team:
http://googleblog.blogspot.com/2005/01/preventing-comment-spam.html
external
external is used to tell the browser to open this link in a new window. However JavaScript must be used to achieve this. But PR is still transferred 🙂