In this post, Alex Golesh talks about the launchers and choosers mechanism in Windows Phone 7 applications.
The Windows Phone application model isolates every application in its own sandbox, both for execution and file storage. Applications are not able to directly access common stores of information, such as the contacts list to directly invoke other applications such as the phone or messaging applications. To enable scenarios that require common tasks such as these, Windows Phone exposes a set of APIs referred to as Launchers and Choosers that allow applications indirect access to these useful phone features. The Launcher and Chooser APIs invoke distinct built-in applications that replace the currently running application.