sql question after upgrade

Hi just upgraded to 7.3.2 and imported my db schema with no probs!

Just wondering... why VOLATILE is being appended to my functions - can anyone ansewer me what this means? - should i be using joins instead, does this make a difference? If so an example of this would be much appreciated!!

CREATE FUNCTION public.get_people_total(int4) RETURNS int8 AS '
select count(person_id) as total from person where person_id IN
(
select cp.person_id from cluster_person cp, cluster c where cp.cluster_id = c.cluster_id and c.c_id = $1 group by cp.person_id
);
' LANGUAGE 'sql' VOLATILE;