A Sitemap index is an XML file that lists multiple XML sitemap files. The XML format of a Sitemap index file is very similar to the XML format of a Sitemap file. It allows webmasters to include additional information about each XML sitemap (when it was last updated). After creation of a Sitemap index file webmasters can notify search engines about the index file, and the other XML sitemaps that are included in the Sitemap index file will be automatically notified too... see online examples

Submitting Sitemaps

Also, the location of the Sitemap index can be specified using a robots.txt file to help search engines find the Sitemap index files. To do this, the following lines need to be added to robots.txt:

Google uses an XML schema to define the elements and attributes that can appear in Sitemap index file. In order to validate your Sitemap or Sitemap index file against a schema, the XML file will need additional headers.

Generate XML Sitemap Now
With this tool you can directly notify Google, Yahoo!, Ask.com, Bing (Live.com) and Moreover.com about your new or updated sitemap file.