simple linking question

I'm trying to do some more advanced things in sql to make my tables more
efficient. I'd like to do a link, or perhaps it's a join, although
everything I've looked at relating to a join makes me think that it's not
what I'm looking for.

What I would like to do is have a field in a table retrieve its data from
a field in another table. The idea is not to have duplicated information
in two different tables. Is this possible to do with postgres? I studied
join selects, but it seems this method requires that information be
duplicated in a third table. I want to avoid duplicating data.

Or is this just stupid?

Thanks for any help or pointers to documentation.

-jeremy

--
salad.