Sitemaps are an easy way for webmasters to inform search engines about pages on their sites that are available for crawling. By creating and submitting Sitemaps to search engines, you are more likely to get better freshness and coverage in search engines.
Google Sitemap Generator is a tool installed on your web server to generate the Sitemaps automatically. Unlike many other third party Sitemap generation tools, Google Sitemap Generator takes a different approach: it will monitor your web server traffic, and detect updates to your website automatically.