A few years ago, search engines started using links and link popularity to affect placement in their indexes. And of course, as soon as spammers figured this out, they started trying to find ways to get around it or improve their rankings artificially. One way to do this is to add the links themselves - in the comments.
The major search engine providers (Google, Yahoo!, and MSN) got together and came up with a way to fight this form of link spam - the rel="nofollow" attribute. And now, most blog software programs add this attribute to links in comments automatically.
What Does the rel="nofollow" Attribute Do?
When a search engine spider comes to a page with links on it, the standard response is to index the page and then follow all the links on the page. That is when the search engine looks at the links to see which ones you have "voted" for by putting them on your page. When the search engine spider sees "nofollow" on the link, it:
- not follow the link to the new site
- not count the link towards it's popularity score in their ranking engine
- not include the link text in the relevancy score for those keywords
Search engines will not penalize your site or blog for links that have nofollow on them. It also doesn't prevent a site from ever being indexed in Google. And putting nofollow on links does not imply any judgement about those links. In other words, if you apply the nofollow attribute to a link you aren't saying that you think it's bad or spam, just that the link isn't yours and shouldn't be followed by search engines.
Why Do You Want Nofollow Links on Comments
The most important reason to add nofollow to your comment links is to discourage spammers. No, this won't stop them altogether (I don't think anything stops them), but it will discourage them a little bit. As while they can get their link on your page (assuming it isn't filtered by a spam filter also on your site), the link itself won't help them beyond the possibility that someone might click on it.
You don't have to rely only on your blog software to flag comment links. You can use the attribute anywhere on your site you want to. The best places to use it are on external links that you want to include on your site but don't want to send to search engines, places like forums, chat, and other user generated content most often.
