What and why?

Sitemaps are an easy way for webmasters to inform search engines about pages on their sites that are available for crawling. In its simplest form, a Sitemap is an XML file that lists URLs for a site along with additional metadata about each URL (when it was last updated, how often it usually changes, and how important it is, relative to other URLs in the site) so that search engines can more intelligently crawl the site.

Web crawlers usually discover pages from links within the site and from other sites. Sitemaps supplement this data to allow crawlers that support Sitemaps to pick up all URLs in the Sitemap and learn about those URLs using the associated metadata. Using the Sitemap protocol does not guarantee that web pages are included in search engines, but provides hints for web crawlers to do a better job of crawling your site.

The protocol is offered under an Attribution-ShareAlike Creative Commons License, so it can be used by any search engine, derivative variations using the same license can be created and it can be used for commercial purposes.

People who use Google Sitemaps don’t need to change anything, because it has already use the protocol described in sitemaps.org, only now those maps will now be indexed by Yahoo and Microsoft.

If you're using WordPress, you might like to use the sitemap generator plug ins here. I've been using it for couple months, and Google has indexed these pages beautifully.