postgres: list role grants for all users
You can see the users using the psql client with the option \du+(like @dezso already explained).

But, if you want it, you can make a query on pg_roles, eg:

  SELECT 
      r.rolname, 
      r.rolsuper, 
      r.rolinherit,
      r.rolcreaterole,
      r.rolcreatedb,
      r.rolcanlogin,
      r.rolconnlimit, r.rolvaliduntil,
  ARRAY(SELECT b.rolname
        FROM pg_catalog.pg_auth_members m
        JOIN pg_catalog.pg_roles b ON (m.roleid = b.oid)
        WHERE m.member = r.oid) as memberof
, r.rolreplication
, r.rolbypassrls
FROM pg_catalog.pg_roles r
ORDER BY 1;


Please, take a look at the documentation for more details:
https://www.postgresql.org/docs/current/static/user-manag.html