Application Server domains are logical or physical units created to help the administrator manage a system configuration. A domain is broken down into smaller units including instances and node agents. A server instance is a single Java Virtual Machine (JVM) that runs the Application Server on a single physical machine. Each domain has one or more instances. A domain must also have at least one associated node agent for the instance to function properly. Domains can be grouped together to create a cluster. Clusters allow the administrator to manage groups of hardware and software.