actually, jelly fish don't bite... but they do sting :P
the feeling is like being electrecuted and acid burn combined.

as per my experience, if you just leave it, the sting marks will disappear with time.

however, if you cannot bear the initial pain, i think putting some vinegar and rubbing with beach sand will help a bit ;) if that does'nt work, ask for medical help of course.

ohhh, this is a yearly problem. so take care on the beaches during september to december time, since jellyfish are in bloom at this time of the year.