What is a Sitemap file and why should I have one ?
Site Map
In general, there are two types of sitemaps:
- The first type of sitemap is a HTML page listing the pages of your site - often by section - and is meant to help users find the information they need.
- The second type are XML Sitemaps - usually called Sitemaps, with a capital S - are a way for you to give search engines more information about your site.
See http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=40318
Site Map Index
If you have a lot of pages and only some change frequently then a sitemapindex file is useful
You can list the URLs that change frequently in a small number of Sitemaps and then use the lastmod tag in your Sitemap index file to identify those Sitemap files. Search engines can then incrementally crawl only the changed Sitemaps.