Model-View-ViewModel is an architectural pattern mainly used in WPF, Silverlight and WP7 development whose aim is to virtually remove all the code-behind from the View layer. Interactive Designers can focus on UX needs using XAML and create bindings to the ViewModel, which is written and maintained by application developers.
MVVM is a specific implementation of the more general Presentation Pattern. MVVM ViewModel comprises of conceptual models rather than data models, and all the business logic and other operations are done within the Model and the ViewModel. There are many Frameworks which enable this.
Continue reading at InfoQ: MVVM Frameworks For .NET.