Microsoft Expression Blend is a little hard to describe. It's a design tool for creating XAML application interfaces, mainly for programs that run on .Net 3.0 and the WPF (Windows Presentation Foundation). It's been superficially compared to Microsoft's take on Adobe Flash, but we're not sure that's a precise comparison.
Microsoft's stated goal with Microsoft Expression Blend is to allow users to create application front ends that are slick and powerful in a way that Microsoft's programming tools really haven't allowed until now, and in that respect Blend is probably the most genuinely adventurous of the Expression products.
If you're not a programmer, or not intending to write programs that use Microsoft's Silverlight/.Net/XAML/WPF axis of technologies, Microsoft Expression Blend is not likely to be of much use to you. If you are interested in building such things, though, it's certainly worth a look.
When you fire up Microsoft Expression Blend, you're greeted with a workspace that does seem to owe a couple of debts to Flash: Among the panels that are available are event timelines, for creating behaviours that can be hitched to actions such as clicking an object. You can easily switch between Blend's graphical design view and editing the underlying XAML code, if you want to dig into the guts of the project you're working on and make changes by hand.
Integrating existing .Net code into a Microsoft Expression Blend project isn't terribly tough, and Blend supports either C# or Visual Basic on a per-project basis. Blend also comes with a slew of vector design tools that hearken directly back to Design (and to other vector drawing programs before it), and it loads not only XAML objects but, interestingly enough, Wavefront 3-D objects and textures as well.
The projects you create in Microsoft Expression Blend can run as stand-alones or can be further expanded on in a programming environment such as Microsoft Visual Studio 2005 - especially useful if you're writing a lot of back-end code that needs to be debugged in detail, separately. Programmers who are already familiar with the .Net "code-behind" philosophy that separates the program logic from its visualisation should be able to pick up Blend pretty quickly.
If you're curious about what's possible with Microsoft Expression Blend more or less out of the box, check out the sample projects bundled with the program: a primitive animation studio, a 3D object demo, a "virtual photobook" (complete with turnable pages), a video shelf (like the photobook, but with video), and a playable grand piano. Obviously they're highly simplified examples, but can be taken apart as an example of how to build applications in Microsoft Expression Blend.