I am little bit confuse regarding robots.txt .I want to know what is best condition to use this file, any body can explain to me .
Basically the purp0se of a robots.txt file is that you can prevent the Search Engine crawler from crawling these pages and getting them indexed, this will then prevent them from showing in the Search Engines search results. The pages that I would put in a robots.txt file would the printer friendly pages, the admin pages that shows the admin login and admin logout and any other page that I would not want to see in the results page.
If your site contain page like career.html and there's no need to have it indexed, so you can use the robots.txt file to tell the search engine spider to ignore it.
At the time of onpage optimization, I am cresting the Robot.txt file. I agree with "unknownpray"'s statement.