For Windows Phone applications, tombstoning is the process in which some data about the application state are saved upon app termination so that this state is re-created if the user navigates back to the tombstoned application (in order to simulate multitasking).
From the MSDN article about the Execution Model for Windows Phone, tombstoning is:
The process in which an application is terminated, but some data about the state of the application and the individual pages within the application is preserved. The data that is saved includes the currently displayed application page and the back stack of pages that had previously been visited in the application. If the user navigates back to a tombstoned application, the application is re-created, and the current page and page history are restored automatically.
Concerning the "tombstoned" state of an application:
A tombstoned application has been terminated, but information about its navigation state and state dictionaries populated by the application during Deactivated are preserved. The device will maintain tombstoning information for up to five applications at a time. If an application is tombstoned and the user navigates back to the application, it will be relaunched and the application can use the preserved data to restore state. Otherwise, the application is simply terminated.
