In very simple terms --
It is a vector based set of routines that uses XAML (markup language). It is the direction Microsoft has stated it is going with forms presentation in managed applications.
It's a lot more than just a GUI library, that's for sure. Take a look at the Wikipedia entry: http://en.wikipedia.org/wiki/Windows...ion_Foundation
The Wikipedia entry has some concentrated details that are probably not important in a "what is WPF" query, but it also has headings that give you an idea of what it encompasses.
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