A sitemap.xml file is a list of the URLs (the locations of the pages) of a website; this list is provided to search engines so that they can get to every bit of the content in a website. The sitemap.xml file can also contain information about each such URL, including the date it was last updated, how often it is usually changed, and your own indication of how important you think the page is (compared to all the other pages of your website). Separate sitemaps can be created for images. A sitemap.xml file tells Google (and other search engines) where your pages are so that they can be indexed. Compare to robots.txt file, which tells Google (and other search engines) what NOT to index from your website.
