The Solution Strategy
In a nutshell, the problem is that if you enable the links in comments added by others to your blog, or even just link their name to their site, you're inevitably inviting unscrupulous spammers to add their own garbage posts purely to gain a link from your site to their own.
To see how simple the nofollow solution is, let's peek at some HTML. A hypertext link in HTML looks like this:
<a href=" some URL ">text that's linked</a>
The change that the three big search engines have implemented is the support for a new attribute called rel with a specific value of nofollow. The previous link would be blindly followed by a search engine crawler (for example, Googlebot), and the linked site would gain some PageRank from the source site. With this new attribute, however, links are not followed by the spiders and PageRank is not transmitted. Here's how that link would look in this brave new world:
<a href=" some URL " rel="nofollow" >text that's linked</a>
Six Apart, the company that makes Movable Type, has announced that they've already updated TypePad and that LiveJournal is going to implement the nofollow attribute for comments from people who aren't friends. Other weblog and related systems that have announced support include Blogger, WordPress, Flickr, Buzznet, Scripting News, blojsom and Blosxom.
For Movable Type users, there's a new nofollow plug-in that you'll need to download and install. Fortunately, it's only 4KB total (a tiny Perl script, actually), and it's worth doing right now, while you're thinking about it.