First read about robots.txt at http://en.wikipedia.org/wiki/Robots_exclusion_standard and you can refer to this forum's robots.txt section for more specific discussions: http://forums.digitalpoint.com/forumdisplay.php?f=50
Robots.txt file is just an easy text file store in the web server's root index. It t tells the web spiders not to crawl the web pages of the site.
Robots.txt is a spider protocol file which is allows search engine which Web pages of your site should be indexed and which Web pages should be ignored
Most sites contain pages that should not be indexed by the search engines. Administrative pages, for example, ABC's "contact" page: "contact.html". There's no need to have it indexed, so we use the robots.txt file to tell the search engine spider (robot) to ignore it. http://www.robotstxt.org/robotstxt.html Here given link will help you a lot, All the information regarding robots.txt is available.