Alex Golesh on "tombstoning" →
To enable seamless navigation by limiting the phone to run one application at a time, Windows Phone activates and deactivates applications dynamically, exposing events to developers to respond to when the application state changes. By implementing handlers for these events, developers can save and restore application state as their application transitions between active and inactive states. This behavior creates an experience in which it seems to the user like the application continued to run in the background.