"It lets you see up-to-the-second social updates, news articles and blog posts about hot topics around the world" This is what Google has to say about its offering called RealTime Search. This has a very good feature where in an email alert can be created for social updated or new topic discussion happening around the world and in outerspace ;) One example use case would be to set an alert such that when somebody mentions your twitter handle you can get an instant email alert.
To set up ,go to http://www.google.com/realtime then type in your handle for me it is @brainchunk. Google would populate the real time results. Now scroll to the bottom of the page where you can see "Create an email alert for @brainchunk", Click on this and you would be redirected to Google's another service Google Alert, here set your preferences. That's it. Next time some one mentions your handle you would get an instant notification. This is just one use case of Google Alert. You can Create many other Alerts as well.