XAML is an XML-based language used to define user interfaces for Windows applications. It allows developers to declaratively create user interface elements and define their layouts. XAML is similar to HTML in some ways, as both use markup tags to define UI elements, but XAML is designed for desktop and Windows applications while HTML targets web pages. Some key similarities between XAML and HTML include common elements like buttons, text boxes and lists. However, XAML has advantages for Windows apps by tightly integrating with .NET and offering richer controls compared to basic HTML elements. The document discusses the purpose and usage of both XAML and HTML, comparing their features and providing examples of common elements in each.