how to refer to tables in another database( or schema as oracle refers to)

Hello,
I'm still new here and new to PostgreSQL, I'd like to know that after I 
have created two databases on my site, such as one is called backend, and 
another one is called admin, how do I refer(query) the table from backend 
while I'm connected to admin database, or is it possible to do that in 
PostgreSQL? any answer is appreciated.

JJW.
8/19/2002