Site Map files are a new standard for search engines. You can create an XML file as part of your site, search engine crawlers can find them, and the file helps define pages and their relation to each other
Each Sitemap file that you provide must have no more than 50,000 URLs and must be no larger than 10MB
If you need to provide more than 50,000 URLs or the file goes over 10MB, you must provide more than one sitemaps file. You will then need to list each Sitemap file in a Sitemap index file. Sitemap index files may not list more than 1,000 Sitemaps and must be no larger than 10MB.