Expressions is more than just another web site editor. It is a set of 3 programs (graphic design, interactive design, web design) that is designed to work with the new Microsoft Presentation Foundation (WPF). WPF will be the UI component that Vista uses when it ships. It`s a complete redesign of how Microsoft products will do graphical presentation.
WPF is based on a markup language called XAML. It`s similar to HTML, and lets graphic designers construct any UI - web page, windows applications, etc -in terms of how graphics components are laid out in relation to each other. A wide variety of components will be supported - vector based graphics, animations, video, etc. Also, all these objects can be managed by using .Net code, which will allow designers to do some pretty dynamic things.
The best way to think of it as .Net meets Flash meets HTML. It`s a long overdue re-write of the Microsoft graphics layer. More info can be found at: