System tables

I'm working on a project which needs to introspect the structure of a
database. I need to fish out table names, attribute names and
type, and foreign keys.

I had a poke about the system tables, but the meaning of the data isn't
always 100% clear. In particular, I could find no easy way to determine the
foreign key constraints (I know that Postgres emulates foreign keys with
rules and triggers).

Does anyone have some definitive information about the contents of the
system tables. Something like a description of the attribute meanings and
an ER diagram explaining table links would be very helpful, and would lead
to faster more accurate hitchhiking.

Cheers

Chris