All XAML elements are an XML representation of CLR classes, but not all CLR classes are represented in XAML. Most of those represented are user-interface elements and are ultimately derived from System.Windows.UIElement, which provides basic visual user-interface properties that are shared by most XAML elements . A System.Windows.UIElement can render itself, receive input via the keyboard and mouse, visually size and position its child elements, and raise events.

Not all XAML elements are derived from System.Windows.UIElement. Some, such as LineBreak, TableColumn, and Document, are derived from System.Windows.Frame-workContentElement. System.Windows.FrameworkContentElement elements cannot render themselves but are instead rendered by another class, usually the container in which they have been placed.
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XAML generally follows XML syntax rules, just as any other XML-based markup language does. Each XAML element has a name and one or more attributes. Attributes correspond directly to object properties, and the name of the XAML element exactly matches the name of a CLR class definition.

XAML is pure markup, which means that while the names of event handlers are specified as attributes, you must implement the actual logic of the event handler in code. If you’re familiar with ASP.NET programming techniques, then you’ll be familiar with the term codebehind, which refers to the code “behind” a XAML interface element that is responsible for providing application logic such as event handlers. It can be implemented in either C# or VB.NET. In both cases, the code can be placed inline in the XAML file, although this contradicts best practices in separating the presentation and application logic layers.
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Microsoft’s new declarative language for defining application user interfaces, XAML stands for eXtensible Application Markup Language. XAML provides an easily extensible and localizable syntax for defining user interfaces separated from application logic, similar to the object-oriented technique for developing n-tier applications with a MVC (Model-View-Controller) architecture.

XAML was created by Microsoft expressly for the purpose of interfacing with its .NET Framework on its Windows Vista (formerly codenamed “Longhorn”) operating system through the WinFX (codename “Avalon”) presentation subsystem runtime environment. XAML gives developers the ability to control the layout of all .NET user-interface elements such as text, buttons, graphics, and listboxes, using XML . Because XAML is XML-based, your code must be well-formed XML. Every XAML tag corresponds directly to a .NET Framework class whose properties are controlled through the use of XML attributes.
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