trust http://www.postgresql.org/docs/7.4/interactive/auth-methods.html#AUTH-TRUST
When trust authentication is specified, PostgreSQL assumes that anyone who can connect to the server is authorized to access the database as whatever database user he specifies (including the database superuser). This method should only be used when there is adequate operating system-level protection on connections to the server.

Ident-based authentication
The ident authentication method works by inspecting the client's operating system user name and determining the allowed database user names by using a map file that lists the permitted corresponding user name pairs. The determination of the client's user name is the security-critical point, and it works differently depending on the connection type.


В чем разница ? И там и там если системно авторизовался то можешь подключится к бд, еси такой пользователь есть в бд. а там если такой пользователь есть в мап файле.