Vaadin is a Java-based web application framework that allows building rich client-side web applications using server-side Java. It provides over 350 reusable UI components and supports major browsers. Vaadin handles communicating between the server-side Java code and browser-based JavaScript, allowing developers to write code once that works across all devices without needing browser plugins. Vaadin embraces Java and runs on any JVM, supporting technologies like Servlets, Portlets, and most cloud platforms.