In mid 2005, the search engine Google launched the Sitemap 0.84 Protocol which would use the XML format to list each page within a web site. The Google's XML format was designed for the search engines, allowing them to find the data faster and more efficiently.
Through this XML protocol, Google and now most other search engines can track the URLs more efficiently. This aids them in optimizing their search by placing all the information in one page. XML also lists how frequently a particular website is updated, and records the last time any changes were made to the code of the web page.
XML sitemaps are not a tool for search engine optimization, and does not affect ranking, but it allows the search engines to index your site more deeply that makes for a more accurate rankings and searches. It does this by providing the basic data that a search engine needs by putting it one place and in a standard format.
What is "XML sitemap"?
By placing an XML formatted file with all you site's links enable Search Engine crawlers (like Google) to find out what pages are present and which have recently changed, and to crawl your site accordingly.
What is "Change frequency"?
Change frequency indicates how often the content of particular web page is likely to change.
What is "Last Modified"?
The Last Modified date is when the code of the URL was last modified. This information allows crawlers to avoid re-crawling your documents if they haven't changed.
What is "Priority"?
The Priority of a URL relative to the other pages in the site. The value for this tag is a number between 0.0 and 1.0, where 0.0 identifies the lowest priority page(s) on your site and 1.0 identifies the highest priority page(s) on your site. The default priority of a page is 0.5.