Vaadin (Finnish for doe, a female reindeer) is a Java framework for building web applications. Unlike many other frameworks out there, in Vaadin you develop your front-end from the server side out, or more specifically, you use a server-side Java API to build the front-end layout (using Swing-like layout managers) and behaviour of your application. The demo video on the Vaadin website will make this more clear:
You can learn more about the framework from the on-line book hosted at the Vaadin website:
Updated 2022 : note that the above post was originally published in 2011, but is left here for archival purposes. I have fixed some broken links above.