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