Question: I don't quite understand what the mean by this (MSDN):

`XAML directly represents the instantiation of objects in a specific set of backing types defined in assemblies. This is unlike most other markup languages, which are typically an interpreted language without such a direct tie to a backing type system.`

What is a backing type and backing type system? Are these sets file where tags for the XAML are defined?

Answer: 
`Backing Types` here just means that there are specific, direct equivalent classes that match your XAML declarations. Your XAML is the markup - but the WPF assemblies have exactly matching classes that "back those up".

This is as opposed to HTML; your web browser probably doesn't have a class named `img` to match your `<img>` tags. It actually might; but HTML is not meant to represent distinct, backing types. XAML, on the other hand, is *meant* to have a 1-to-1 relationship between its object tags and actual classes in code.
Roughly speaking, XAML is a language for describing the instantiation of an object tree and setting of various properties. As such, it's very closely related to the CLR and its type system. It's extremely powerful and it's used extensively by WPF, though there is nothing preventing you from using it for other types of object trees.

XAML is made 'WPF specific' by using this namespace definition:

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

This tells the XAML parser that the default namespace should be as defined above, which then maps to all of the various WPF class types. This URL is defined by the various assemblies that contain WPF-specific types by defining one or more XmlnsDefinitionAttribute on each assembly, mapping the various assembly namespaces to the XML namespace.

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For additional information on the meaning of XAML, see [XAML Syntax In Detail](http://msdn.microsoft.com/en-us/library/ms788723.aspx). There is also a link there to the XAML language specification. It's worth noting that XAML is a compiled language, the product of which is a .NET class that can be instantiated. It is a language that is designed for efficiently describing deeply nested and complex content, with property value assignments and addition of elements to collections, which can be awkward to do from C#. The XAML compiler does support code-behind, however, which allows you to associate additional C# code with the class constructed by the XAML.