The back button is a key feature differentiating Windows Phone from the iPhone and Android.The hardware back button is mandatory on all Windows Phones and will likely be a key navigation feature in Windows Phone applications.
The back button is used to navigate back within an application. Normally developers do not have to worry about implementing this function as, by default, the framework performs this operation. However developers can write custom code to perform the back operation. One such example is a context menu. A context menu is simply a pop-up menu that overlays an active page. To do this the BackKeyPress event has to be handled on the OnBackKeyPress method.