Extensions
Extensions are modules commonly used to install additional features on a database server.
Foreign data wrappers
Foreign-data wrappers are objects used to consume data from different data sources, not necessarily another PostgreSQL database system.
Foreign servers
A foreign server encapsulates connection information that a foreign data wrapper uses to access an external data source.
Foreign tables
Foreign tables, like tables, are containers that store several kinds of other objects, like columns, constraints, and triggers.
User mappings
The user mapping object typically encapsulates connection information that a foreign data wrapper uses together with the information encapsulated by a foreign server to access an external data resource.