Vaadin Framework is a Java web application development framework that is designed to make creation and maintenance of high quality web-based user interfaces easy. Vaadin supports two different programming models: server-side and client-side. The server-driven programming model is the more powerful one. It lets you forget the web and program user interfaces much like you would program a desktop application with conventional Java toolkits such as AWT, Swing, or SWT. But easier.
Vaadin uses a client-side engine for rendering the user interface of a server-side application in the browser. All the client-server communications are hidden well under the hood. Vaadin is designed to be extensible, and you can indeed use any 3rd-party widgets easily, in addition to the component repertoire offered in Vaadin. In fact, you can find hundreds of add-ons in the Vaadin Directory.
Vaadin allows flexible separation between the appearance, structure, and interaction logic of the user interface. You can design the layouts either programmatically or declaratively, at the level of your choosing. The final appearance is defined in themes in CSS or Sass, as described in "Themes".