What is XAML?

Extensible Application Markup Language; pronounced "zammel") XAML is a declarative XML-based language that defines objects and their properties in XML. XAML syntax focuses upon defining the UI (user interface) for the Windows Presentation Foundation (WPF) and is therefore separate from the application code behind it. Although XAML is presently for use on the Windows platform, the WPF/E (Windows Presentation Foundation/Everywhere) initiative will eventually bring XAML to other platforms and devices.

http://www.xaml.net/