using Robots.txt is a file we can guide search engines to crawl or not to crawl certain sections of your website.

Actually the search engines handle nofollow differently. Google will not follow the link, assign PR to it, or give it weight in its algorithm. Yahoo will follow the link but give it no weight in their algorithm. MSN says they won't give the link any value but it is hard to confirm exactly what they're doing.

Well ... robots.text isa text file in which you difine instruction which page you want to be crawl or not to all robots of search engines.

Robots.txt is a text file you put it on your web site to tell search robots which pages you would like them not to visit. It is important to clarify that robots.txt is not a way from preventing search engines from crawling your site.