a basic question about WPF
See the question and my original answer on StackOverflowx is a prefix for an XML namespace. Here is the official doc on this: XAML Namespaces and Namespace Mapping for WPF XAML
If you want more on the XML namespace topic (unrelated to XAML): http://wap.w3schools.com/xml/xml_namespaces.asp
StaticResource is a "Markup extension", here is the official doc again: StaticResource Markup Extension