Prism provides guidance designed to help you more easily design and build rich, flexible, and easy to maintain Windows Presentation Foundation (WPF) desktop applications and Silverlight Rich Internet Applications (RIAs) and Windows Phone 7 applications.
Using design patterns that embody important architectural design principles, such as separation of concerns and loose coupling, Prism helps you to design and build applications using loosely coupled components that can evolve independently but which can be easily and seamlessly integrated into the overall application. Such applications are known as often referred to as composite applications.
You can get the v4 update from the patterns & practices team at Prism 4.0 - November 2010 from Microsoft Downloads.
Prism includes reference implementations, QuickStarts, reusable library code (the Prism Library), and extensive documentation. This version of Prism targets the Microsoft .NET Framework 4.0 and Silverlight 4 and includes new guidance around the Model-View-ViewModel (MVVM) pattern, navigation, and the Managed Extensibility Framework (MEF). Because Prism is built on the .NET Framework 4.0 (which includes WPF) and Silverlight 4, familiarity with these technologies is useful for evaluating and adopting Prism.
Bruce D. Kyle
ISV Architect Evangelist | Microsoft Corporation