Database roles are conceptually completely separate from
   operating system users. In practice it might be convenient to
   maintain a correspondence, but this is not required. Database roles
   are global across a database cluster installation (and not
   per individual database). To create a role use the CREATE ROLE SQL command:
CREATE ROLE name;
   name follows the rules for SQL
   identifiers: either unadorned without special characters, or
   double-quoted.  (In practice, you will usually want to add additional
   options, such as LOGIN, to the command.  More details appear
   below.)  To remove an existing role, use the analogous
   DROP ROLE command:
DROP ROLE name;
For convenience, the programs createuser and dropuser are provided as wrappers around these SQL commands that can be called from the shell command line:
createusernamedropusername
   To determine the set of existing roles, examine the pg_roles
   system catalog, for example
SELECT rolname FROM pg_roles;
   The psql program's \du meta-command
   is also useful for listing the existing roles.
  
   In order to bootstrap the database system, a freshly initialized
   system always contains one predefined role. This role is always
   a “superuser”, and by default (unless altered when running
   initdb) it will have the same name as the
   operating system user that initialized the database
   cluster. Customarily, this role will be named
   postgres. In order to create more roles you
   first have to connect as this initial role.
  
   Every connection to the database server is made using the name of some
   particular role, and this role determines the initial access privileges for
   commands issued in that connection.
   The role name to use for a particular database
   connection is indicated by the client that is initiating the
   connection request in an application-specific fashion. For example,
   the psql program uses the
   -U command line option to indicate the role to
   connect as.  Many applications assume the name of the current
   operating system user by default (including
   createuser and psql).  Therefore it
   is often convenient to maintain a naming correspondence between
   roles and operating system users.
  
The set of database roles a given client connection can connect as is determined by the client authentication setup, as explained in Chapter 21. (Thus, a client is not limited to connect as the role matching its operating system user, just as a person's login name need not match his or her real name.) Since the role identity determines the set of privileges available to a connected client, it is important to carefully configure privileges when setting up a multiuser environment.