Outer Joins

How are outer joins implemented within PostgreSQL?
I would like to do a query of the form:
   select a.x, b.y from a, b where a.x *= b.x;

The *= operator does not seem to exist.  I've been
able to devise some clumsy workarounds by creating
temporary tables:
   create table temp_table as      select a.x, b.y from a, b where a.x = b.x;
   insert into temp_table      select x from a      except          select a.x from a, b where a.x = b.x;

Outer joins are apparently quite common, but I haven't
been able to find anything on this in either the users
guide, the draft of Bruce Momjian's upcoming book or the
Deja News archives.

Charles Mott