Microsoft has released a White Paper on the topic Introduction to Background Tasks in Windows 8 Release Preview which is now available for download from Microsoft Download Center.
Windows 8 Release Preview was released few weeks back and includes lot of new functionalities.
One of the main features of the Windows 8 Release Preview is the Metro Style Apps.
This whitepaper explains the programming model to create Background tasks and built-in controls that allow the user to control per-app background task using the lock screen personalization.
As a prerequisite, you must be familiar with the Metro Style app development in Windows 8.
Download White Paper – Windows 8 Release Preview – Introduction to Background Tasks @ Microsoft Download Center