postgres ident error

[Tom Allison <tallison@tacocat.net>]

stuck.

I created a user in psql but now I can't log in as that user.

I get an error
psql: FATAL:  IDENT authentication failed for user "dbmail"

This implies that I need to have all my users for pgsql listed in both 
the /etc/passwd file and the postgres database.  Or does postgres just 
us the /etc/passwd file for all of it's authentications?

I'm curious as to why this is done.