Google Sitemaps are XML files that list the URLs available on a site. The aim is to help site owners notify search engines about the URLs on a website that are available for indexing.
Webmasters can include information about each URL, such as when it was last updated and its importance in the context of the site.
A site map (or sitemap) is a list of pages of a web site accessible to crawlers or users. It can be either a document in any form used as a planning tool for web design, or a web page that lists the pages on a web site, typically organized in hierarchical fashion. This helps visitors and search engine bots find pages on the site.
While some developers argue that site index is a more appropriately used term to relay page function, web visitors are used to seeing each term and generally associate both as one and the same. However, a site index is often used to mean an A-Z index that provides access to particular content, while a site map provides a general top-down view of the overall site contents.
XML is a document structure and encoding standard used, amongst many other things, as the standard for WebCrawler