A rich client platform (RCP) is a framework of widgets, widget containers and other processing capabilities that facilitate sophisticated application development and deployment. One important aspect of RCPs is that they tend to be highly cross-platform because platform support is provided by the framework.
In both cases, you get sophisticated capabilities and cross-platform support. In the case of RCPs, you get these benefits by learning a framework, deploying the platform to the client-side and developing applications targeted for the platform. The one negative here is that you are invested heavily in the platform, so if some aspect of the platform is a showstopper (such as large download size), then you won't be able to deliver the application. In the case of RIAs, you get the benefits by learning a framework (browser HTML/JS/AJAX + J2EE), deploying the platform to the server side and developing applications targeted for the platform. The one negative here is that you are heavily invested in the platform, so if some aspect of the platform is a showstopper (such as no offline operation of the application), then you won't be able to deliver the application.
XForms solves the RIA/RCP conundrum by providing a framework for expressing the core data processing asset of an XML-based application independently of the deployment paradigm. There are XForms implementations of both types, RCP and RIA. In particular, the IBM Lotus Forms Webform Server is a product that converts an XForms-based Lotus Form into a rich internet application (RIA) automatically. The IBM Lotus Forms Viewer is a rich client platform (RCP) that provides the run-time for XForms-based Lotus Forms. The application developer is free to think about the application without thinking so much about the pecularities of how it will be deployed.