The Extensible Application Markup Language (XAML) by Microsoft is a new declarative programming language DESIGNED to create user INTERFACES for managed applications. WPF uses XAML to construct GUI (Graphical User Interface) for Windows applications. XAML is a markup language designed for creating user interfaces that look like XML (Extensible Markup Language).  XAML is optional (in TERMS of usage), but it serves as the core of the WPF UI design. 

Advantages of XAML-

• Designing UI elements with XAML is another simple and easy task.
• The purpose of XAML is to allow visual designers to directly design user interface elements.
• In ADDITION to designing GUIs, it can also be used for other purposes, for example, to create workflows in Workflow Foundation.
• With XAML, you can easily create, initialize, and specify properties for objects with hierarchical relationships.
• Through XAML, WPF lets you control all visual aspects of the user interface.
• Both bitmap and vector images are supported.
• In XAML, visible UI elements can be created and separated from programming logic.