Notice: MVVM Light is no longer actively developed or maintained and will be removed as an option in a future version of Windows Template Studio.
A guide is available if you want to use the new MVVM Toolkit and are migrating from MvvmLight.
MVVM Light is a popular, 3rd party framework based on the Model-View-ViewModel pattern by Laurent Bugnion. The MVVM Light Toolkit helps you to separate your View from your Model which creates applications that are cleaner and easier to maintain and extend. It also creates testable applications and allows you to have a much thinner user interface layer (which is more difficult to test automatically).
This toolkit puts a special emphasis on the "blend ability" of the created application (i.e. the ability to open and edit the user interface into Blend), including the creation of design-time data to enable the Blend users to "see something" when they work with data controls.
You can find more out at MVVM Light's homepage