So far, you have been introduced to the most common parts of a Vaadin application. Vaadin is mostly about using an API with Java running on the server side. This Java code defines how the application looks and behaves, but a Vaadin application runs on a browser using HTML 5 and JavaScript. You don't have to write a line of HTML or JavaScript in order to implement a Vaadin application. How is this possible? How does a Java class define the HTML rendered in the browser?

The key to understanding this is the widget set. A widget set is a JavaScript engine running on the client side, which contains all the code required to show components and communicate with the server side. A widget set is generated by compiling a set of Java classes into JavaScript using GWT. These Java classes are provided by the Vaadin Framework and you can add your own if you want to. If you are not using custom client-side components (your own, or those provided by a third-party Vaadin add-on), you can use the already compiled widget set which is included in the vaadin-client-compiled dependency.