Apache Geronimo consists of an IoC kernel that provides dependency management, configuration management, lifecycle management, and repository services. The plumbing required by Java EE containers, namely, support for deployment, transactions, remoting, naming, work management, and security, is implemented on top of the kernel. On top of this are plugged in the various containers, such as Apache OpenEJB, Apache ActiveMQ, and so on, which provide parts of the Java EE functionality and together make Geronimo a Java EE application server.

GBeans are the basic building blocks of Geronimo. A GBean provides lifecycle callbacks and acts as a thin wrapper around the various containers and modules that are plugged into Geronimo. The kernel then manages these containers and modules via the GBeans that wrap them. A group of GBeans that wraps a particular module is called a configuration. Interestingly, a configuration itself is a GBean, thereby leading to the statement, "Everything in Geronimo is a GBean!!"

The Geronimo architecture is modular and extensible. The extensibility is achieved via plugins. Geronimo provides the ability for new functionality to be added as plugins. Plugins are actually configurations or groups of configurations, along with metadata about their dependencies. Plugins can be installed in Geronimo via the Administration Console or via the command-line tools. Geronimo has the ability to download and install plugins from a remote repository. One such remote repository is available at http://geronimoplugins.com/. Thus, the Geronimo kernel also provides plugin installation and management services. Each of the containers in Geronimo is actually installed as a plugin. This method of extending the features of Geronimo via plugins initially gave rise to a minimal version of the server called little Geronimo (Little-G) that consisted of the Geronimo kernel, the required plumbing, and the web container. Later, even more flexibility was provided and you could export servers containing only the components you needed, from the Administration Console. This provided a large amount of flexibility in the configurations. A set of predefined server profiles were also provided out of the box, for users to export.