I'm curious about this since many website use rel=nofollow and I don't know why.If you have a site that's ranked very good in Google, and you don't add rel=nofollow to links inside article comments, how does that affect your rank?What if someone posts a comment with a link to a viagra website? Does that lower the rank of your site?
About rel=nofollow inside links
google;nofollow
You can read a bit of the background history of nofollow here:  http://en.wikipedia.org/wiki/NofollowBasically the intent is to reduce the incentive to spam links to your site on other people's blogs/comment sections.  If the hosting site marks every link in a comment as nofollow, then the spamming site doesn't get any additional ranking from the spammed links.  And that helps remove the incentive to spam the links in the first place.If you operate the hosting site and you don't use nofollow on comment links and someone links to a questionable site, then it's hard to say if your ranking can be negatively affected.  It would be up to the search engine's ranking algorithm to decide, really.  But as far as I know, at least Google will penalize you for linking to low-quality (as in, link-farms and the like) sites.