Attaching information about users

What is the proper way to attach additional information about users of
a database. That is to say I am using their DB login as their application
ID, and I need to have one or more tables which remember preferences and
other application level things, but the key is their current userid and
I want to be sure that I only have entries for people who are currently
users in this DB. I suppose what I want to do is to use foreign key
constraints, but that would be to a postgresql specific table (I guess,
or is the table that holds the list of IDs and its field names common
across DBs?).

David