Extensible Application Markup Language (XAML) is a declarative language..
XAML is just XML that Microsoft has developed to describe objects. Silverlight relies on XAML to store and represent visual objects to be displayed and animated on HTML pages. XAML is a XML file which defines the UI elements. This XML file can be read by WPF framework or Silver light framework for rendering. Microsoft first developed WPF and they used XAML files to describe the UI elements to the WPF framework. Microsoft then extended WPF and made WPF/E which helped to render the UI in the browser. WPF/E was the code name for Silverlight. Later Microsoft launched Silverlight officially.