When you log into your webmaster tools page you will see it has had a small face lift. Instead of a list of your sites it now has snazzy little thumbnails.
Google have also added another nifty little function, the Site Health report. If Google hasn’t found anything to worry about it will display the text “nothing to report” next to the thumbnails of your sites.
If though it has detected something you need to address it will display a Site Health alert icon 
At the moment the things that will trigger the alert are :
Google detects malaware on your site
An important page is removed from your site
An important page is blocked by robots.txt
Another good reason why you should be using Google Webmaster Tools.
You might also like to read :
Dashboard - Overview of the dashboard in Google Webmaster Tools
Site Configuration - read what the site configuration section covers.
Your Site on the Web - read about the Your Site on the Web section.
+1 Metrics - What is the Google +1 and how Google reports back to you
Diagnostics - Read what’s in the diagnostic section.
Labs - Google playground.
