Null...

PG7.1.2 on RH7.0

select 'a'<>null;
 ?column?
----------

(1 row)

select 'a'!=null;
 ?column?
----------

(1 row)

select 'a'=null;
 ?column?
----------
 f
(1 row)

select 'a'||null;
 ?column?
----------

(1 row)

Can someone please explain the above?