Google Webmaster Tools provide you with detailed reports about your website’s visibility on Google. You can use the Tools to see exactly how Google’s “bots” or “spiders” see your site, which allows you to diagnose problems that may hamper your organic search engine rankings. By fully understanding how Google crawls and indexes the pages on your site, you can learn about specific problems Google may have while accessing it.
You can also use the Tools to understand your link and query traffic. Using Google’s link reporting tools, you can view, organize, and download detailed information about the internal and external links to your site. You can also determine which specific Google search queries drive the highest volume of traffic to your site, and see the exact path that users have taken to arrive at your site.
The Tools also allow you to share information about your site with Google. You can submit your sitemap to help Google better understand your site, which pages are most important to you, and how often they may change. You can also let Google know how you would like the URLs they index to appear.
For more information on Google’s Webmaster Tools, see http://www.google.com/webmasters/