WPF markup extensions

is called a markup or markup tag
{StaticResource ...} is called a markup extension

Markup extension enables the XAML parser to translate value to primitives or objects. There two major categories of extensions: XAML-Defined Markup Extensions and WPF-Specific Markup Extensions

XAML-Defined Markup Extensions
x:Type
x:Static
x:Null
x:Array

WPF-Specific Markup Extensions (Listed 3 most important extensions below)
StaticResource
DynamicResource
binding

http://msdn.microsoft.com/en-us/library/ms747254.aspx