In the following documentation, site may be interpreted as the host machine on which Postgres is installed. Since it is possible to install more than one set of Postgres databases on a single host, this term more precisely denotes any particular set of installed Postgres binaries and databases.
The Postgres superuser is the user named postgres who owns the Postgres binaries and database files. As the database superuser, all protection mechanisms may be bypassed and any data accessed arbitrarily. In addition, the Postgres superuser is allowed to execute some support programs which are generally not available to all users. Note that the Postgres superuser is not the same as the Unix superuser (which will be referred to as root). The superuser should have a non-zero user identifier (UID) for security reasons.
The database administrator or DBA, is the person who is responsible for installing Postgres with mechanisms to enforce a security policy for a site. The DBA can add new users by the method described below and maintain a set of template databases for use by createdb.
The postmaster is the process that acts as a clearing-house for requests to the Postgres system. Frontend applications connect to the postmaster, which keeps tracks of any system errors and communication between the backend processes. The postmaster can take several command-line arguments to tune its behavior. However, supplying arguments is necessary only if you intend to run multiple sites or a non-default site.
The Postgres backend (the actual executable program postgres) may be executed directly from the user shell by the Postgres super-user (with the database name as an argument). However, doing this bypasses the shared buffer pool and lock table associated with a postmaster/site, therefore this is not recommended in a multiuser site.