http://schemas.microsoft.com/winfx/2006/xaml/presentation is the core Silverlight namespace. It encompasses all the essential Silverlight classes, including the UserControl and Grid. Ordinarily, this namespace is declared without a namespace prefix, so it becomes the default namespace for the entire document. In other words, every element is automatically placed in this namespace unless you specify otherwise.
http://schemas.microsoft.com/winfx/2006/xaml is the XAML namespace. It includes various XAML utility features that allow you to influence how your document is interpreted. This namespace is mapped to the prefix x. That means you can apply it by placing the namespace prefix before the name of an XML element or attribute.
< x:ElementName > and x:Class=”ClassName”