Take a look at Windows.Phone.UI.Input.HardwareButtons.
If you add a Basic Page to your project then VS will add a NavigationHelper class to your project which helps with Navigating thru your App, you can also see in the source code that it is subscribing to Windows.Phone.UI.Input.HardwareButtons.BackPressed
.
In case you want to extend handling the Back Button (managing Eventhandler queue and so on) you may take a look at this answer - the method there will help to prevent backward navigation (if you don't need it) and add some more behaviours.