Windows Presentation Foundation (WPF) is Microsoft's new software layer for producing and managing user interfaces. For the technical amongst you it vectorises graphical data and converts it into XAML (pronounced 'zaml').
To author in this new format Microsoft have released a number of packages under the 'Expression' name. Expression Web is, in all but name, a new version of the html editing package FrontPage. The most interesting product is Expression Blend - a competitor for Adobe Flash.
In order to make this work across platforms and browsers, another leaf has been taken from Flash's book with a small 2MB plug-in enabling a reasonably seamless experience - this is called WPF/E (for everywhere).
You can check out some examples here.
There are some nice examples, but there's little here that wasn't present in Macromedia Director over a decade ago. However, Microsoft have played catch up very well and, with the existing benefits of WMV over Flash video such as better quality, DRM and live events, the empire has struck back.