The robots.txt file
When a Robot visits a Web site, say
http://www.foobar.com/, it firsts checks for
http://www.foobar.com/robots.txt. If it can find this document, it will analyze its contents to see if it is allowed to retrieve the document. You can customize the robots.txt file to apply only to specific robots, and to disallow access to specific directories or files.
Here is a sample robots.txt file that prevents all robots from visiting the entire site
User-agent: * # applies to all robots
Disallow: / # disallow indexing of all pages