A sitemap is a list of pages on a web site that are available to visitors or search bots and crawlers. These sitemaps are usually arranged in a hierarchical manner.
The document structure and encoding standard usually used as the standard for webcrawlers to find and parse sitemaps is XML.
The sitemap protocol is a map of the site that allows search engines to crawl a site more effectively. Webmasters can use the sitemap protocol to include additional information about each URL on the site. In particular, when the site was last updated, how often the site is updated, and the importance of each url on the site relative to other URLs.
The sitemap is very important for internet marketing / search engine optimization (SEO).
Sitemaps supplement but do not replace the existing crawl-based mechanisms that search engines use to discover URLs.