Different RDBMS (relational database management systems) have different terminology when it comes to clusters, instances, databases and schemas.
In DBmarlin we try to take a generic definition of these terms and apply it to all RDBMS platforms that we support.
A cluster is a group of instances that are related. Typically they will be related through Master-Slave replication (Active Passive) or Multi-Master replication (Active Active). This could be done for high availability (HA), disaster recovery (DR), geo-replication or scaling out.
An instance is the software installed and memory used to access a database. Typically this corresponds to a host/port combination where the port is the listener port for the database software.
Database or Schema
A database or schema, depending on the RDBMS used, defines how the data will be organised (into tables and other objects) and contains the data and metadata stored in physical files on disk.