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 website, typically organized in hierarchical fashion.
Sitemaps make relationships between pages and other content components. It shows shape of information space in overview. Sitemaps can demonstrate organization, navigation, and labeling system.
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. HTML sitemaps are not supported in Google Webmaster Tools.
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 glance.
Google introduced the Sitemaps protocol 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 contains URLs to these pages so that web crawlers can find them. Bing, Google, Yahoo and Ask now jointly support the Sitemaps protocol.
Since Bing, 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. Google Webmaster Tools allow a website owner to upload a sitemap that Google will crawl, or they can accomplish the same thing with the robots.txt file. On 1st Dec 2016 both Bing and Google confirmed that they now support xml sitemaps up to 50MB in size while uncompressed.
XML Sitemaps have replaced the older method of "submitting to search engines" by filling out a form on the search engine's submission page. Now web developers submit a Sitemap directly, or wait for search engines to find it. Regularly submitting an updated sitemap when new pages are published allows search engines to find and index those pages more quickly than it would by finding the pages on its own .
XML (Extensible Markup Language) is much more precise than HTML coding. Errors are not tolerated, and so syntax must be exact. It is advised to use an XML syntax validator such as the free one found at: http://validator.w3.org
More information defining the field operations and other Sitemap options are defined at http://www.sitemaps.org (Sitemaps.org: Google, Inc., Yahoo, Inc., and Microsoft Corporation).
Below is an example of a validated XML sitemap for a simple three page web site. Sitemaps are a useful tool for making sites built in Flash and other non-html languages searchable. If a website's navigation is built with Flash, an automated search program would probably only find the initial homepage; subsequent pages are unlikely to be found without an XML sitemap.
XML sitemap example:
<?xml version="1.0" encoding="UTF-8"?> <urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"> <url> <loc>http://www.example.net/?id=who</loc> <lastmod>2009-09-22</lastmod> <changefreq>monthly</changefreq> <priority>0.8</priority> </url> <url> <loc>http://www.example.net/?id=what</loc> <lastmod>2009-09-22</lastmod> <changefreq>monthly</changefreq> <priority>0.5</priority> </url> <url> <loc>http://www.example.net/?id=how</loc> <lastmod>2009-09-22</lastmod> <changefreq>monthly</changefreq> <priority>0.5</priority> </url> </urlset>
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