From VGInterface Wiki
An XML schema is the file that documents what content associated XML files can (or must) possess. An application can then validate and process the content for the use required (such as a User Interface in an MMORPG).
The schema for Vanguard's User Interface(UI) can be found in your Vanguard/VGUIAssests folder with the name vanguard-ui.xsd.
Elements and Attributes
The UI schema consists almost solely of XML elements. Elements act much in the same way that classes do in other programming languages. They contain declarations of what types the element consists of. The XML validation program ensures that when an element is used in the file, that it conforms to this declaration.
Attributes are Simple Types that define properties of the parent element. For example, many elements use the Name attribute:
In this example, the name assigned to this VGUIWindow element is Grafts.
Each element can be of a simple type, or a complex type. A Complex Type allows the element to use a mixture of elements and attributes to further define the element.