No Follow

From Link Building Wiki

In early 2005 Blog spam came to a head when the search engines realized it was clogging up the index.

Through automated scripts many sites were able to garner hundreds or thousands of links through submitting automated comments in blogs.

As such the engines banded together and promoted a new tag attribute which could be easily added to blog comments forcing the search engine crawlers to not follow the link found in the comment.

While this type of blog spam is still common it's use seems to have dropped off substantially.

by simply adding rel="nofollow" to a comment url, the search engine spiders would understand that such links are not to be followed.

Official Google Blog post on comment spam

Yahoo blog post - A Defense Against Comment Spam

MSN Search Weblog - http://blogs.msdn.com/msnsearch/archive/2005/01/18/nofollow_tags.aspx

Search Engine Watch - provides a nice summary of the whole nofollow attribute

Wikipedia Entry on NoFollow Attribute - An interesting look at Nofollow as well as how Wikipedia dealt with the issue.