Extensible Application Markup Language a.k.a XAML
<Button>Click me! </Button>
The above example looks more like a markup language. If it is so, why do we need a new markup language? I found an interesting explanation in the inside XAML article from ONDOTNET website.
HTML is primarily targeted towards web and it doesn’t provide support for rich user experience, whereas XAML’s principal target is applications that run directly on Windows (i.e., "rich clients" or "smart clients").
The most powerful reason for Microsoft to devise a new markup is the very close relationship between elements in a XAML file, and objects at runtime. Unlike any previous markup languages, XAML is designed to integrate directly with WinFX.
More information here….