Default Users for Postgres
The default User is created during installation of the server instance. But you can query for a list of all current Users using this query:
SELECT usename AS role_name,
  CASE 
     WHEN usesuper AND usecreatedb THEN 
       CAST('superuser, create database' AS pg_catalog.text)
     WHEN usesuper THEN 
        CAST('superuser' AS pg_catalog.text)
     WHEN usecreatedb THEN 
        CAST('create database' AS pg_catalog.text)
     ELSE 
        CAST('' AS pg_catalog.text)
  END role_attributes
FROM pg_catalog.pg_user
ORDER BY role_name desc;

For more information please see this article.
You can change the Users and their passwords using ALTER USER, as discussed here. You can also disable the login for the default User as stated here.