Foreign keys?

There doesn't seem to be much documentation on foreign keys and how to
use them properly.

I am setting up a DB with two tables: users, and a table of objects
where are always owned by users.  I want to make sure that there's no
way to have an object which isn't owned.  Let's say these are the
tables:

CREATE TABLE user (
       number INT4,
       name VARCHAR(400)
);

and

CREATE TABLE object (
       owner INT4,
       description VARCHAR(200)
);

The constraint is that there should never be a row in the object table
where the owner column has a number which doesn't have a corresponding
owner in the user table.

I'm sure I can do something with foreign keys to implement this
constraint, but I can't figure it out.

Thanks