Google Sitemaps are intended to inform and direct search engine crawlers to indexing your site page by page.

Using this approach search engine spiders could expand their coverage of the web and speed up the discovery and addition of pages to their index.

Sitemap is a XML formatted file on your web server which tells search engines what pages should be indexed and an optional priority is added for each link (0-1).

for more information please visit:
Google Site Map Exmplained

google also provided their own sitemap generator at:
Google Sitemap Generator