An XML (Extensible Markup Language) Sitemap is a text file used to detail all URLs on a website. It can include extra information (metadata) on each URL, with details of when they were last updated, how important they are and whether there are any other versions of the URL created in other languages. All of this is done to help the search engines crawl your website more efficiently, allowing any changes to be fed to them directly, including when a new page is added or an old one removed.