Transitionals is a framework for building and using WPF transitions which provide an easy way to switch between UI views in a rich and animated way. Think of transitions for applications in the same way you think of transitions for video editing. Wipe, Cut, Dissolve, Star, Blinds and 3D Rotating Cube are all examples of transitions supported by the Transitionals framework.
The best way to get started with the Transitionals framework is to download and take a look at the TransitionalsHelp file. You can find it on the Releases tab and it includes a pretty comprehensive Getting Started guide. You can also download the binary archive which includes two sample projects. Transitionals is compiled in Visual Studio 2008 against .Net Framework 3.5.