If you’ve been following the blogosphere for a while, you know that there was a huge problem with comment spam around 2003 or so. The problem arose because most blogs and online guestbooks allow you to supply a link to your own blog when you add a comment. Spammers realized that they could use automated robots to add comments and therefore links to the sites they were being paid to promote.
For a while the comment areas of certain blogs were almost unusable, and many bloggers were overwhelmed with the number of comments they had to delete. Exacerbating this problem was the fact that it didn’t matter if any person actually followed these spam links, it only mattered if the search engines found them. This is because search engines determine the relative importance of web sites by counting how many other sites link to them.
Naturally it was the folks at Google (and Blogger) who responded to this problem by proposing a slight change to the way links are handled. Understanding how that works requires a little background.