There is some specialized terminology associated with BigchainDB. To get started, you should at least know the following:
A BigchainDB node is a machine or set of closely-linked machines running MongoDB Server (or RethinkDB Server), BigchainDB Server, and related software. Each node is controlled by one person or organization.
A set of BigchainDB nodes can connect to each other to form a BigchainDB cluster. Each node in the cluster runs the same software. A cluster contains one logical MongoDB/RethinkDB datastore. A cluster may have additional machines to do things such as cluster monitoring.
The people and organizations that run the nodes in a cluster belong to a BigchainDB consortium (i.e. another organization). A consortium must have some sort of governance structure to make decisions. If a cluster is run by a single company, then the "consortium" is just that company.
What's the Difference Between a Cluster and a Consortium?
A cluster is just a bunch of connected nodes. A consortium is an organization which has a cluster, and where each node in the cluster has a different operator.