PostgreSQL consists of a set of server processes, the group leader of which is named the postmaster. Starting the server is the act of creating these processes, and stopping the server is the act of terminating those processes.
Each postmaster listens for client connection requests on a defined port number. Multiple concurrently running postmasters cannot share that port number. The port number is often used to uniquely identify a particular postmaster and hence also the database server that it leads.
When we start a database server, we refer to a data directory, which contains the heart and soul—or at least the data—of our database. Subsidiary tablespaces may contain some data outside the main data directory, so the data directory is just the main central location, and not the only place where data for that database server is held. Each running server...