Couchbase Server can be used either in a standalone configuration, or in a cluster configuration where multiple Couchbase Servers are connected together to provide a single, distributed, data store.
In this description:
Couchbase Server or Node
A single instance of the Couchbase Server software running on a machine, whether a physical machine, virtual machine, EC2 instance or other environment.
All instances of Couchbase Server are identical, provide the same functionality, interfaces and systems, and consist of the same components.
Cluster
A cluster is a collection of one ore more instances of Couchbase Server that are configured as a logical cluster. All nodes within the cluster are identical and provide the same functionality. Each node is capable of managing the cluster and each node can provide aggregate statistics and operational information about the cluster. User data is is stored across the entire cluster through the vBucket system.
Clusters operate in a completely horizontal fashion. To increase the size of a cluster, you add another node. There are no parent/child relationships or hierarchical structures involved. This means that Couchbase Server scales linearly, both in terms of increasing the storage capacity and the performance and scalability.