Official Google Blog: Controlling how search engines access and index your website
I’m often asked about how Google and search engines work. One key question is: how does Google know what parts of a website the site owner wants to have show up in search results? Can publishers specify that some parts of the site should be private and non-searchable? The good news is that those who publish on the web have a lot of control over which pages should appear in search results.
The key is a simple file called robots.txt that has been an industry standard for many years. It lets a site owner control how search engines access their web site. With robots.txt you can control access at multiple levels — the entire site, through individual directories, pages of a specific type, down to individual pages. Effective use of robots.txt gives you a lot of control over how your site is searched, but its not always obvious how to achieve exactly what you want. This is the first of a series of posts on how to use robots.txt to control access to your content.