There are two popular versions of a site map. An XML Sitemap is a structured format that a user doesn’t need to see, but it tells the search engine about the pages in a site, their relative importance to each other, and how often they are updated. HTML sitemaps are designed for the user to help them find content on the page, and don’t need to include each and every subpage. This helps visitors and search engine bots find pages on the site. You cannot submit an HTML sitemap in Google Webmaster Tools as it is not a supported sitemap format.
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 alphabetically-organized access to particular content, while a site map provides a general top-down view of the overall site contents organized with a classification system.
They also act as a navigation aid  by providing an overview of a site’s content at a single
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