A PostgreSQL instance is called a cluster because a single instance can serve and handle multiple databases. Every database is an isolated space where users and applications can store data.
A database is accessed by allowed users, but users connected to a database cannot cross the database boundaries and interact with data contained in another database, unless they explicitly connect to the latter database too.
A database can be organized into namespaces, called schemas. A schema is a mnemonic name that the user can assign to organize database objects, such as tables, into a more structured collection. Schemas cannot be nested, so they represent a flat namespace.
Database objects are represented by everything the user can create and manage within the database—for instance, tables, functions, triggers, and data types. Every object belongs to one and only one schema that, if not specified, is the default public schema.
Users are defined at a cluster...