A sitemap is a representation of the architecture of a web site. 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.
Benefits of sitemaps
Most search engines will only follow a finite number of links from a page, so if a site is very large, the site map may be required so that search engines and visitors can access all content on the site.
Google Sitemaps are most popular to use, so web developers can publish lists of links from across their sites. The basic premise is that some sites have a large number of dynamic pages that are only available through the use of forms and user entries. The sitemap files can then be used to indicate to a web crawler how such pages can be found.
Since MSN, Yahoo, Ask, and Google use the same protocol, having a sitemap lets the four biggest search engines have the updated page information. Sitemaps do not guarantee all links will be crawled, and being crawled does not guarantee indexing. However, a sitemap is still the best insurance for getting a search engine to learn about your entire site.
XML sitemaps have replaced the older method of "submitting to search engines" by filing out a form on the search engines submission page. Now web developers submit a sitemap directly, or wait for search engines to find it.