In Windows 8, C# will be used in writing applications or invoking Jupiter-based applications on the Jupiter platform.

What is Jupiter?

Jupiter is a programming platform that Microsoft will use to leverage applications for both Windows 8 and Windows Phone in the near future. It is a new user interface library for Windows. Although different from Silverlight or WPF, it will be fully compatible with both, and will use IE 10’s rendering engine. Using a XAML-based framework, it will provide access to Windows 8 elements, like sensors and networking. Jupiter apps will be packaged as AppX application types that could be common to both Windows 8 and Windows Phone 8. The AppX format is universal, and can used to deploy native Win32 apps, framework-based apps (Silverlight, WPF), Web apps, and games.

What is AppX?

AppX is a tight specification that describes how applications are packaged and installed, and was likely inspired by Silverlight’s XAP files.
Although Windows 8 will change many components of applications like how they are written and distributed, AppX is only concerned with the deployment aspect of applications.
AppX packages require an XML manifest file named AppXManifest.xml. In this file, developers will have to specify many compulsory and optional attributes of their application, which is much more than the current requirements in Windows Phone 7.