An XML sitemap is an XML file that lists all URLs of website intended to be crawled and indexed by Google and other search engines. The XML sitemap may also include additional informations for each listed URL through associated metadata which describe when a page has been last updated, how often it is changed or how a page is important relative to other URLs in the site. A sitemap can be also used for giving informations about specific types of content like videos, images or mobile content - see mobile sitemap for more informations.
XML sitemaps are based on Sitemaps protocol which is a URL inclusion protocol and are placed at the root of the web place or webserver. The sitemap may also be submitted through Google Webmaster Tools. A sitemap file can’t contain more than 50,000 URLs and must be no larger than 50 MB. For large websites a sitemap can be broken up in a set of smaller sitemaps.
An XML sitemap does not directly affect search rankings but is important for SEO as it allows search engine spiders to crawl a site more efficiently and ensures webpages indexing.
XML sitemaps should not be confused with HTML sitemaps.
A short video explanation of XML sitemaps :