Microsoft Expression Studio provides a set of professional design tools for .NET web, desktop, WPF or Silverlight applications.
This package contains Expression Web, Expression Blend, Expression Design and Expression Encoder and SketchFlow tools.
For instance, your professional designer can design a rich UI using Expression tools and produce a XAML result. Then your professional WPF developer gets the XAML and develops code and logic for it.
To build websites; similar to DreamWeaver and FrontPage but with more features and supports AJAX, C#, etc.
Using which you can create rich UI (e.g. 3D) for .NET, WPF and Silverlight.
Using which you can build designs and vector assets (similiar to Photoshop but with different features)
For instance you can publish videos from your webcam to Silverlight. You can also customize your videos; adding titles, etc.
It is a great tool for prototyping.
For a full list of features, see this.