The world wide web is basically nothing more than a bunch of pages linked together by hyperlinks (awesome name, that). Larry Page realized the importance of these links when he came up with the PageRank algorithm, the foundation of Google search.
By linking to your favorite site you are essentially giving it your vote. Crawlers can then pick up on this to figure out which sites are popular and which ones contains the most relevant information.
You can mark some of your links with the “nofollow” property, which effectively says that you do not want to give that site your vote. This is a good way to link to sites that are related to your information but which you do not want to condone.
Fighting spammers
It is also used to prevent spammers from posting jibberish comments to your pages. By posting this kind of spam, ill-willed individuals can build a lot of illegitimate votes to their own sites. If you add nofollow to these links then you have effectively removed their incentive.
This is a double edged sword however. When you use nofollow for all the comments on your site then you are also disrupting the very foundation of the Internet. Perfectly valid links are discarded and what could be a healthy, flowing ecosystem ends up in single, disconnected entities.
Personally I find the spammer argument weak. Pretty much every blog out there use nofollow for their comment’s links and they are still being attacked. You can seldom run a site with user comments without employing a spam protection service, like Akismet or Mollom.
Therefor we are left only with the downsides of this strategy. Spammers should be fought but this is not the weapon for it. So let us stop this folly and instead help grow the Internet the way it was meant.
Remove those obsolete nofollow rels!