Everything you need to know about Google Sitemaps

Usually Google's crawler Googlebot will find each and every page on your web site, as long as a link known by Google points to it. Once Googlebot has spidered a page, it returns every once in a while to check for your updates. Shortly after Googlebot's visit, Google updates its index and changes the ranking of your pages. If you've changed the content (visible text, image descriptions...) of a page, Google will deliver your pages on its SERPs for new keywords or it will rank your pages differently. If you've added new pages and if you provide links to these pages, Google includes them into its index. That's pretty much simplistic, but to make use of Google's SiteMap service you don't need to understand the details.