what is "null"

I execute "select max(kind_id) into maxTypeid from prod_kind where kind_id like '77' || '__';" sql statement and no data returned. What is the maxTypeid's value? I think it's null value. But I use this statement to judge:
if maxTypeid = null then
dbms_output.put_line('no data');
else
dbms_output.put_line('a' || maxTypeid || 'b');
end if;
This block statement returned "ab". How can I judge maxTypeid value?
Please help
thanks

Don't use maxTypeid = null never, because

null = null --> FALSE

right statment :

if maxTypeid is null then
dbms_output.put_line('no data');
else
dbms_output.put_line('a' || maxTypeid || 'b');
end if;

oh,oh. It's right. thanks very much.