rel="nofollow"

Not wanting search engine robots to follow links and include then in an index. See also the noindex META tag.

The Google support forum describes the use of this attribute, briefly:

• Untrusted content
• Paid content
• Crawling Priority - wanting Google and other search engines to crawl you pages and
  not ones you have linked to.

An example:

<a href="http://sample-site.co.uk" rel="nofollow"><a>

Using a CMS may or may not give you control over this. Wordpress has a Plugin (Akismet to discourage "Comment Spam".

The effect on SEO

One of the theories that I had in the past was the effect of outbound links from my websites. The theory was that all you were doing was giving away Page Rank to the site you are linking. The Page Rank of your own pages is cited as being dependant on the websites linking to your page.