Hmm I have no idea. We were involved in car clubs when I was a kid. All the members had to own this particular kind of car, no others were accepted.

So I'm guessing that a club has a more restricted membership whereas with an association people can join if they have an interest in that particular area.

For example, a 4x4 club would require you to own a 4x4 and an association would require you to have a keen interest.