Fron Google - http://www.google.com/support/webmasters/bin/answer.py?answer=96569&ctx=sibling "Nofollow" provides a way for webmasters to tell search engines "Don't follow links on this page" or "Don't follow this specific link." Originally, the nofollow attribute appeared in the page-level meta tag, and instructed search engines not to follow (i.e., crawl) any outgoing links on the page. For example: <meta name="robots" content="nofollow" />Before nofollow was used on individual links, preventing robots from following individual links on a page required a great deal of effort (for example, redirecting the link to a URL blocked in robots.txt). That's why the nofollow attribute value of the rel attribute was created. This gives webmasters more granular control: instead of telling search engines and bots not to follow any links on the page, it lets you easily instruct robots not to crawl a specific link. For example: <a href="signin.php" rel="nofollow">sign in</a>
Default it is dofollow. But if you want that some pages should not be indexed by Google than you can upload an Robots.txt file and mentioned in the file that you do not want to indexed these pages, For example.... User-agent: * Disallow: /apple.domain.com Disallow: /banana.domain.com Disallow: /orange.domain.com You can also use nofollow attribute to prevent passing benefits of PR to any link. Such as, <a href="abc.htm" rel="nofollow">ABC</a>
Do follow allows the spider to follow your page and nofollow attribute tells the search engine spider to don't follow the link. To see if the link is a dofollow, right click on the link and click properties. If it says new window, it means dofollow. If it says external, it is a dofollow. If there is an external nofollow, it is a nofollow. It if there is a nofollow, it is a nofollow.
There is no such thing as a Dofollow attribute. "Dofollow" is simply the absense of the nofollow attribute. Nofollow: <a href="someurl.htm" rel="nofollow">text</a> "Dofollow": <a href="someurl.htm">text</a>