Libgda is composed of three independent layers. The lower level is covered by the GDA providers, which are plug-ins whose task is to map the RDBMS-specific API to the GDA model. That is, they are objects implementing the GDA interfaces.
Then, in a middle layer, are the client libraries: an easy-to-use and full featured library which offers access to all the architecture, also including several utility functions to help you on the development of applications based on GDA. This library, although targeted to client applications, also includes a set of helper classes and functions to help in the addition of new providers to the Libgda architecture.
Finally, at the upper level sit all the client applications provided in the suite, as well as any application that may make use of the client libraries.