Source: www.adventive.com/tools/SEO.html
A text file stored in the top level directory of a web site to deny access by robots to certain pages or sub-directories of the site. Only robots which comply with the Robots Exclusion Standard will read and obey the commands in this file. Robots will read this file on each visit, so that pages or areas of sites can be made public or private at any time by changing the content of robots.txt before re-submitting to the search engines. The simple example below attempts to prevent all robots from visiting the /secret directory: User-agent: * Disallow: /secret