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. The spiders periodically revisit sites to find new or changed content. Google Sitemaps exists to inform and direct Google's crawlers. 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.
Here is a nice PHP tool to automate your Sitemap file creation: http://enarion.net/google/
Read more: Google Sitemaps