WPF is the engine that is responsible for creating, displaying, and manipulating user-interfaces, documents, images, movies, and media in Windows Vista.

Physically, WPF is a set of libraries that have all functionalty you need to build, run, execute, and manage Windows Vista applications.

Its just not the matter of presentation, Microsoft introduced the whole new plumbing.

More maintainable, more achievable and more powerful than previous frameworks.

One major thing is that, we are using 10 to 15 years old window api(user32). it has been replaced in WPF, a new , better , more secure-able and powerful api which could communicate with directX (Indirectly).

More portable technology, you could create component for other type of projects.(winForm, ASP.NET and others ).

Provided common interface between in wpf desktop and browser hosted applications.

One xaml could be used in silverlight and wpf but Silverlight is a subset of WPF.

So Silverlight has less features than WPF.

Developer and Designer could work independently.
Designer could work in Microsoft Design tool as expression blend and expression design.

In previous technologies, developers had to be dependent on designer.

For example: we were developing a flash button which could animate and change its text accordingly.
designer design it in flash tools (or may in action script) and gave us in .gif or some animated formats.

we were limited in change.

Microsoft provides a common and portable solution through xaml (zaml).
Now no need to learn a new language and technology , you could work in a single language.

Moreover, Microsoft introduced most powerful properties Dependency Property and Attached Property.

Everything magically happened through above two properties.

Get a detail overview.

http://msdn.microsoft.com/en-us/library/aa970268.aspx
http://msdn.microsoft.com/en-us/library/ms754130.aspx