A sitemap is a file, usually in XML format, that lists the pages of a website along with metadata about each page (last modification date, update frequency, relative priority). Search engines read sitemaps to discover and index content more efficiently, especially for large sites or pages that are not well-linked from the rest of the site.
The sitemap protocol was jointly adopted by Google, Yahoo, and Microsoft in 2006. A site’s robots.txt file typically includes a reference to the sitemap location so that crawlers can find it automatically.