什么是Qt Widget？

Qt帮助文档如此解释：

The user interface contains visual elements that are called widgets in Qt. Examples of widgets are text edits, scroll bars,
labels, and radio buttons. A widget can also be a container for other widgets; a dialog or a main application window, for
example.

简而言之，widget（译名：部件）就是包含可视化元素的用户接口，如编辑框，按钮，标签等等。一些部件如对话框、主窗口等可以容纳其他部件。