Sitemap are an easy way for people to inform Search Engines about pages that's avaliable on the net.
A sitemap file is an XML format file that list URL's for a site along with additional metadata about each URL.
Data for last updates
How often it changes
How important it is. (Relative to other pages on the same site).
Google, Yahoo and MSN are joining forces to get a united sitemap format. This will save lots of work for webmasters and a morae rapid indexing.