Why dont you Google it rather than ask a stupid 1st question,i can also see you posting alot more of these stupid question just to build post counts up.
Google Webmaster Tools provides you with detailed reports about your pages' visibility on Google. Some of the features are Get Google's view of your site and diagnose problems See how Google crawls and indexes your site and learn about specific problems occurred during accessing it. Discover your link and query traffic View, classify, and download comprehensive data about internal and external links to your site with new link reporting tools. Find out which Google search queries drive traffic to your site, and see exactly how users arrive there. ​Share information about your site Tell google about your pages with Sitemaps: which ones are the most important to you and how often they change. You can also let us know how you would like the URLs we index to appear.
You can learn everything if you reach the official google web masters page. Simply it helps to diagnose problems in your site and monitor the progress of you site.