The presentation layer of the medium example consists of several JSP (Java Server Pages) that rest in the src/webapp directory. The JSPs itself use the Java Server Faces technology that was introduced with Java EE 5. JSF introduces components that can be reused in many scenarios and greatly reduce the often tedious work of writing JSPs. It also defines how events in these web components can be wired to server-side services.
To understand Java Server Faces, please refer to Chapter 9 of the Java EE 5 Tutorial. The API documentation for JSF can be found at: http://www.oracle.com/technetwork/java/javaee/javaserverfaces-139869.html.