A robots.txt file restricts access to your site by search engine robots that crawl the web. These bots are automated, and before they access pages of a site, they check to see if a robots.txt file exists that prevents them from accessing certain pages. (All respectable robots will respect the directives in a robots.txt file, although some may interpret them differently. However, a robots.txt is not enforceable, and some spammers and other troublemakers may ignore it. For this reason, we recommend password protecting confidential information.)
You need a robots.txt file only if your site includes content that you don't want search engines to index. If you want search engines to index everything in your site, you don't need a robots.txt file (not even an empty one).
While Google won't crawl or index the content of pages blocked by robots.txt, we may still index the URLs if we find them on other pages on the web. As a result, the URL of the page and, potentially, other publicly available information such as anchor text in links to the site, or the title from the Open Directory Project (www.dmoz.org), can appear in Google search results.
In order to use a robots.txt file, you'll need to have access to the root of your domain (if you're not sure, check with your web hoster). If you don't have access to the root of a domain, you can restrict access using the robots meta tag.