Authentication Problems

Please suggest me what are all the way i
can implement AUTHENTICATION on USER , DATABASE and TABLE.
As i have seen, to my knowledge i understood we can implement 
Grant and Revoke on table
But how can we do the same on the DATABASE
There is a password attribute in pg_shadow 
how can this be used to implement on USER


And finally please help to know whether Postgres 6.3.2
has security on the USER AND DATABASE

HOW CAN I CREATE pg_ident.conf and how can i use it  for authentication 
on User/Database/Table