Nofollow

Nofollow is a tag you use in HTML code of a Link. If you don't want Googlebot to follow a link and Index it from your site, you can use the following nofollow tag in your href links: rel="nofollow"

Possible uses include channelling of PR away from non-monetary pages such as an "about" page. So, instead of having a live link to the about page, by having the html:

Googlebot will not:
1.Index the page from that link
2. Consider the Anchor text of the link or
3. Pass any PR to the linked page.