NULL v. null string

If NULL is not the same as a null string, then why does this block of code display 'YES'.

begin
  if '' is null then
    dbms_output.put_line('yes');
  else
    dbms_output.put_Line('no');
  end if;
end;

Same this with this. If null string is a known value and NULL is an unknown value, why does this display no?

begin
  if '' = '' then
    dbms_output.put_line('yes');
  else
    dbms_output.put_Line('no');
  end if;
end;

--
Chuck Hamilton
chuck_hamilton_at_yahoo.com

"Do not be deceived, God is not mocked; 
for whatever a man sows, this he will also
reap." (Gal 6:7 NASB)