|Search engines such as Google discover information about your site by employing software known as "spiders" to crawl the web. Once the spiders find a site, they follow links within the site to gather information about all the pages.
If your site has dynamic content or pages that aren't easily discovered by following links, you can use a Sitemap file to provide information about the pages on your site. This helps the spiders know what URLs are available on your site and about how often they change.
A Sitemap provides an additional view into your site (just as your home page and HTML site map do). This program does not replace the spider's normal methods of crawling your web site. A Sitemap gives Google (and other spiders) additional information that they may not otherwise discover. For more information, see the Google Site Maps FAQ.
UPDATED 7/2008: now submits to Yahoo, Ask! & Moreover in addition to Google.
UPDATED 6/2009: now submits to Bing in addition to all the others.