Which license applies to each piece of Open RDF source code is shown on the Downloads page. The full texts of the licenses are available at the following web-sites:
The GPL is of interest for those who want to use Open RDF sources with the Kylix Open Edition, which was itself released under the terms of the GPL. Note that the GPL requires that the complete sources of an application are available under the GPL. Since the sources of the Delphi VCL which are required for each application are not released under the GPL, the GPL is irrelevant for Delphi applications. If you own Delphi or a commercial version of Kylix and want to make your sources available for users of the Kylix Open Edition, you may nevertheless deploy your units under the GPL, too. As long as you release them only under the terms of the GPL these sources must not be used with Delphi or with non-GPL Kylix code.
The MPL is currently the most widespread open source license for Delphi units. So deploying Open RDF sources under the terms of the MPL allows other developers, who use the MPL for their products, to straightforwardly integrate anything from the Open RDF sources into their code. The essentials of this license are in short: If you deploy an application using Open RDF sources (or a modified version thereof) you must make the Open RDF source code (or the code of your modified version) available. It is not required to publish the rest of your application's source code. For details see the full text of the MPL.