From the RAP Project website:
The RAP project enables developers to build rich, Ajax-enabled Web applications by using the Eclipse development model, plug-ins with the well known Eclipse workbench extension points, JFace, and a widget toolkit with SWT API (using qooxdoo for the client-side presentation). The project is part of the Ganymede release train and released its 1.1 release.
One of the key features of RAP is that it allows single sourcing a Rich Web and Rich client from the same codebase.
While RAP is not intend to build websites, it suits for many business cases for internal applications, and even opening them for external access, for example in a business 2 business case. Creating web accessible applications, that can be developed, tested and mainteained just like regular rich client applications brings benefits on many aspects.
The tough technical challenge RAP has overcome, is that it had ro reimplement many of the original Eclipse RCP code. But with clever ideas and a lot of work they managed to provide a nearly complete implementation of RCP for the web.
it is: Bringing Rich Clients to the Web