From EQ2Interface Wiki
What is a Widget?
Widgets are the generic name for all objects that can be used to create a UI page. Widgets include text boxes, dropdowns, check boxes, lists, and even the pages themselves. Every widget has its own set of properties that you can use to manipulate the widget.
Widgets follow an inheritance chain which means that they inherit the functionality of their parent object. So instead of each widget object needing to have a Name or Location property, they inherit these properties from their parent. This way, child widgets only need to implement the specific properties that make them different from their parent.